diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 9a8cf2f8038..c5787c3b29d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -17453,7 +17453,7 @@ namespace ts {
                     assumeTrue = !assumeTrue;
                 }
                 const valueType = getTypeOfExpression(value);
-                if ((type.flags & TypeFlags.Unknown) && (operator === SyntaxKind.EqualsEqualsEqualsToken) && assumeTrue) {
+                if ((type.flags & TypeFlags.Unknown) && assumeTrue && (operator === SyntaxKind.EqualsEqualsEqualsToken || operator === SyntaxKind.ExclamationEqualsEqualsToken)) {
                     if (valueType.flags & (TypeFlags.Primitive | TypeFlags.NonPrimitive)) {
                         return valueType;
                     }
diff --git a/tests/baselines/reference/unknownType2.errors.txt b/tests/baselines/reference/unknownType2.errors.txt
index 5f08804b9e0..1469526182b 100644
--- a/tests/baselines/reference/unknownType2.errors.txt
+++ b/tests/baselines/reference/unknownType2.errors.txt
@@ -228,4 +228,41 @@ tests/cases/conformance/types/unknown/unknownType2.ts(216,13): error TS2322: Typ
         // Arguably this should be never.
         type End = isTrue<isUnknown<typeof x>>
     }
+    
+    // Repro from #33483
+    
+    function f2(x: unknown): string | undefined {
+      if (x !== undefined && typeof x !== 'string') {
+        throw new Error();
+      }
+      return x;
+    }
+    
+    function notNotEquals(u: unknown)  {
+        if (u !== NumberEnum) { }
+        else {
+            const o: object = u;
+        }
+    
+        if (u !== NumberEnum.A) { }
+        else {
+            const a: NumberEnum.A = u;
+        }
+        
+    
+        if (u !== NumberEnum.A && u !== NumberEnum.B && u !== StringEnum.A) { }
+        else {
+            const aOrB: NumberEnum.A | NumberEnum.B | StringEnum.A  = u;
+        }
+    
+        // equivalent to
+        if (!(u === NumberEnum.A || u === NumberEnum.B || u === StringEnum.A)) { }
+        else {
+            const aOrB: NumberEnum.A | NumberEnum.B | StringEnum.A  = u;
+        }
+    }
+    
+    
+    
+    
     
\ No newline at end of file
diff --git a/tests/baselines/reference/unknownType2.js b/tests/baselines/reference/unknownType2.js
index 2ec5b3ed205..cb1b0a58029 100644
--- a/tests/baselines/reference/unknownType2.js
+++ b/tests/baselines/reference/unknownType2.js
@@ -221,6 +221,43 @@ function switchResponseWrong(x: unknown): SomeResponse {
     // Arguably this should be never.
     type End = isTrue<isUnknown<typeof x>>
 }
+
+// Repro from #33483
+
+function f2(x: unknown): string | undefined {
+  if (x !== undefined && typeof x !== 'string') {
+    throw new Error();
+  }
+  return x;
+}
+
+function notNotEquals(u: unknown)  {
+    if (u !== NumberEnum) { }
+    else {
+        const o: object = u;
+    }
+
+    if (u !== NumberEnum.A) { }
+    else {
+        const a: NumberEnum.A = u;
+    }
+    
+
+    if (u !== NumberEnum.A && u !== NumberEnum.B && u !== StringEnum.A) { }
+    else {
+        const aOrB: NumberEnum.A | NumberEnum.B | StringEnum.A  = u;
+    }
+
+    // equivalent to
+    if (!(u === NumberEnum.A || u === NumberEnum.B || u === StringEnum.A)) { }
+    else {
+        const aOrB: NumberEnum.A | NumberEnum.B | StringEnum.A  = u;
+    }
+}
+
+
+
+
 
 
 //// [unknownType2.js]
@@ -388,3 +425,29 @@ function switchResponseWrong(x) {
             throw new Error('Can you repeat the question?');
     }
 }
+// Repro from #33483
+function f2(x) {
+    if (x !== undefined && typeof x !== 'string') {
+        throw new Error();
+    }
+    return x;
+}
+function notNotEquals(u) {
+    if (u !== NumberEnum) { }
+    else {
+        var o = u;
+    }
+    if (u !== NumberEnum.A) { }
+    else {
+        var a = u;
+    }
+    if (u !== NumberEnum.A && u !== NumberEnum.B && u !== StringEnum.A) { }
+    else {
+        var aOrB = u;
+    }
+    // equivalent to
+    if (!(u === NumberEnum.A || u === NumberEnum.B || u === StringEnum.A)) { }
+    else {
+        var aOrB = u;
+    }
+}
diff --git a/tests/baselines/reference/unknownType2.symbols b/tests/baselines/reference/unknownType2.symbols
index 6e871bd4de4..19a1c7fca6e 100644
--- a/tests/baselines/reference/unknownType2.symbols
+++ b/tests/baselines/reference/unknownType2.symbols
@@ -582,3 +582,108 @@ function switchResponseWrong(x: unknown): SomeResponse {
 >x : Symbol(x, Decl(unknownType2.ts, 210, 29))
 }
 
+// Repro from #33483
+
+function f2(x: unknown): string | undefined {
+>f2 : Symbol(f2, Decl(unknownType2.ts, 221, 1))
+>x : Symbol(x, Decl(unknownType2.ts, 225, 12))
+
+  if (x !== undefined && typeof x !== 'string') {
+>x : Symbol(x, Decl(unknownType2.ts, 225, 12))
+>undefined : Symbol(undefined)
+>x : Symbol(x, Decl(unknownType2.ts, 225, 12))
+
+    throw new Error();
+>Error : Symbol(Error, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+  }
+  return x;
+>x : Symbol(x, Decl(unknownType2.ts, 225, 12))
+}
+
+function notNotEquals(u: unknown)  {
+>notNotEquals : Symbol(notNotEquals, Decl(unknownType2.ts, 230, 1))
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+
+    if (u !== NumberEnum) { }
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+
+    else {
+        const o: object = u;
+>o : Symbol(o, Decl(unknownType2.ts, 235, 13))
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+    }
+
+    if (u !== NumberEnum.A) { }
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+>NumberEnum.A : Symbol(NumberEnum.A, Decl(unknownType2.ts, 92, 17))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+>A : Symbol(NumberEnum.A, Decl(unknownType2.ts, 92, 17))
+
+    else {
+        const a: NumberEnum.A = u;
+>a : Symbol(a, Decl(unknownType2.ts, 240, 13))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+>A : Symbol(NumberEnum.A, Decl(unknownType2.ts, 92, 17))
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+    }
+    
+
+    if (u !== NumberEnum.A && u !== NumberEnum.B && u !== StringEnum.A) { }
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+>NumberEnum.A : Symbol(NumberEnum.A, Decl(unknownType2.ts, 92, 17))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+>A : Symbol(NumberEnum.A, Decl(unknownType2.ts, 92, 17))
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+>NumberEnum.B : Symbol(NumberEnum.B, Decl(unknownType2.ts, 93, 6))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+>B : Symbol(NumberEnum.B, Decl(unknownType2.ts, 93, 6))
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+>StringEnum.A : Symbol(StringEnum.A, Decl(unknownType2.ts, 98, 17))
+>StringEnum : Symbol(StringEnum, Decl(unknownType2.ts, 96, 1))
+>A : Symbol(StringEnum.A, Decl(unknownType2.ts, 98, 17))
+
+    else {
+        const aOrB: NumberEnum.A | NumberEnum.B | StringEnum.A  = u;
+>aOrB : Symbol(aOrB, Decl(unknownType2.ts, 246, 13))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+>A : Symbol(NumberEnum.A, Decl(unknownType2.ts, 92, 17))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+>B : Symbol(NumberEnum.B, Decl(unknownType2.ts, 93, 6))
+>StringEnum : Symbol(StringEnum, Decl(unknownType2.ts, 96, 1))
+>A : Symbol(StringEnum.A, Decl(unknownType2.ts, 98, 17))
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+    }
+
+    // equivalent to
+    if (!(u === NumberEnum.A || u === NumberEnum.B || u === StringEnum.A)) { }
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+>NumberEnum.A : Symbol(NumberEnum.A, Decl(unknownType2.ts, 92, 17))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+>A : Symbol(NumberEnum.A, Decl(unknownType2.ts, 92, 17))
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+>NumberEnum.B : Symbol(NumberEnum.B, Decl(unknownType2.ts, 93, 6))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+>B : Symbol(NumberEnum.B, Decl(unknownType2.ts, 93, 6))
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+>StringEnum.A : Symbol(StringEnum.A, Decl(unknownType2.ts, 98, 17))
+>StringEnum : Symbol(StringEnum, Decl(unknownType2.ts, 96, 1))
+>A : Symbol(StringEnum.A, Decl(unknownType2.ts, 98, 17))
+
+    else {
+        const aOrB: NumberEnum.A | NumberEnum.B | StringEnum.A  = u;
+>aOrB : Symbol(aOrB, Decl(unknownType2.ts, 252, 13))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+>A : Symbol(NumberEnum.A, Decl(unknownType2.ts, 92, 17))
+>NumberEnum : Symbol(NumberEnum, Decl(unknownType2.ts, 90, 1))
+>B : Symbol(NumberEnum.B, Decl(unknownType2.ts, 93, 6))
+>StringEnum : Symbol(StringEnum, Decl(unknownType2.ts, 96, 1))
+>A : Symbol(StringEnum.A, Decl(unknownType2.ts, 98, 17))
+>u : Symbol(u, Decl(unknownType2.ts, 232, 22))
+    }
+}
+
+
+
+
+
diff --git a/tests/baselines/reference/unknownType2.types b/tests/baselines/reference/unknownType2.types
index 7fc3c3d1038..ce3332138d5 100644
--- a/tests/baselines/reference/unknownType2.types
+++ b/tests/baselines/reference/unknownType2.types
@@ -627,3 +627,121 @@ function switchResponseWrong(x: unknown): SomeResponse {
 >x : unknown
 }
 
+// Repro from #33483
+
+function f2(x: unknown): string | undefined {
+>f2 : (x: unknown) => string | undefined
+>x : unknown
+
+  if (x !== undefined && typeof x !== 'string') {
+>x !== undefined && typeof x !== 'string' : boolean
+>x !== undefined : boolean
+>x : unknown
+>undefined : undefined
+>typeof x !== 'string' : boolean
+>typeof x : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>x : unknown
+>'string' : "string"
+
+    throw new Error();
+>new Error() : Error
+>Error : ErrorConstructor
+  }
+  return x;
+>x : string | undefined
+}
+
+function notNotEquals(u: unknown)  {
+>notNotEquals : (u: unknown) => void
+>u : unknown
+
+    if (u !== NumberEnum) { }
+>u !== NumberEnum : boolean
+>u : unknown
+>NumberEnum : typeof NumberEnum
+
+    else {
+        const o: object = u;
+>o : object
+>u : object
+    }
+
+    if (u !== NumberEnum.A) { }
+>u !== NumberEnum.A : boolean
+>u : unknown
+>NumberEnum.A : NumberEnum.A
+>NumberEnum : typeof NumberEnum
+>A : NumberEnum.A
+
+    else {
+        const a: NumberEnum.A = u;
+>a : NumberEnum.A
+>NumberEnum : any
+>u : NumberEnum.A
+    }
+    
+
+    if (u !== NumberEnum.A && u !== NumberEnum.B && u !== StringEnum.A) { }
+>u !== NumberEnum.A && u !== NumberEnum.B && u !== StringEnum.A : boolean
+>u !== NumberEnum.A && u !== NumberEnum.B : boolean
+>u !== NumberEnum.A : boolean
+>u : unknown
+>NumberEnum.A : NumberEnum.A
+>NumberEnum : typeof NumberEnum
+>A : NumberEnum.A
+>u !== NumberEnum.B : boolean
+>u : unknown
+>NumberEnum.B : NumberEnum.B
+>NumberEnum : typeof NumberEnum
+>B : NumberEnum.B
+>u !== StringEnum.A : boolean
+>u : unknown
+>StringEnum.A : StringEnum.A
+>StringEnum : typeof StringEnum
+>A : StringEnum.A
+
+    else {
+        const aOrB: NumberEnum.A | NumberEnum.B | StringEnum.A  = u;
+>aOrB : NumberEnum.A | NumberEnum.B | StringEnum.A
+>NumberEnum : any
+>NumberEnum : any
+>StringEnum : any
+>u : NumberEnum.A | NumberEnum.B | StringEnum.A
+    }
+
+    // equivalent to
+    if (!(u === NumberEnum.A || u === NumberEnum.B || u === StringEnum.A)) { }
+>!(u === NumberEnum.A || u === NumberEnum.B || u === StringEnum.A) : boolean
+>(u === NumberEnum.A || u === NumberEnum.B || u === StringEnum.A) : boolean
+>u === NumberEnum.A || u === NumberEnum.B || u === StringEnum.A : boolean
+>u === NumberEnum.A || u === NumberEnum.B : boolean
+>u === NumberEnum.A : boolean
+>u : unknown
+>NumberEnum.A : NumberEnum.A
+>NumberEnum : typeof NumberEnum
+>A : NumberEnum.A
+>u === NumberEnum.B : boolean
+>u : unknown
+>NumberEnum.B : NumberEnum.B
+>NumberEnum : typeof NumberEnum
+>B : NumberEnum.B
+>u === StringEnum.A : boolean
+>u : unknown
+>StringEnum.A : StringEnum.A
+>StringEnum : typeof StringEnum
+>A : StringEnum.A
+
+    else {
+        const aOrB: NumberEnum.A | NumberEnum.B | StringEnum.A  = u;
+>aOrB : NumberEnum.A | NumberEnum.B | StringEnum.A
+>NumberEnum : any
+>NumberEnum : any
+>StringEnum : any
+>u : NumberEnum.A | NumberEnum.B | StringEnum.A
+    }
+}
+
+
+
+
+
diff --git a/tests/cases/conformance/types/unknown/unknownType2.ts b/tests/cases/conformance/types/unknown/unknownType2.ts
index b3d16654b80..7da3093634e 100644
--- a/tests/cases/conformance/types/unknown/unknownType2.ts
+++ b/tests/cases/conformance/types/unknown/unknownType2.ts
@@ -222,3 +222,40 @@ function switchResponseWrong(x: unknown): SomeResponse {
     // Arguably this should be never.
     type End = isTrue<isUnknown<typeof x>>
 }
+
+// Repro from #33483
+
+function f2(x: unknown): string | undefined {
+  if (x !== undefined && typeof x !== 'string') {
+    throw new Error();
+  }
+  return x;
+}
+
+function notNotEquals(u: unknown)  {
+    if (u !== NumberEnum) { }
+    else {
+        const o: object = u;
+    }
+
+    if (u !== NumberEnum.A) { }
+    else {
+        const a: NumberEnum.A = u;
+    }
+    
+
+    if (u !== NumberEnum.A && u !== NumberEnum.B && u !== StringEnum.A) { }
+    else {
+        const aOrB: NumberEnum.A | NumberEnum.B | StringEnum.A  = u;
+    }
+
+    // equivalent to
+    if (!(u === NumberEnum.A || u === NumberEnum.B || u === StringEnum.A)) { }
+    else {
+        const aOrB: NumberEnum.A | NumberEnum.B | StringEnum.A  = u;
+    }
+}
+
+
+
+