diff --git a/packages/quicktype-core/src/language/JavaScript/JavaScriptRenderer.ts b/packages/quicktype-core/src/language/JavaScript/JavaScriptRenderer.ts
index 0adcaefd30..c14a44efdb 100644
--- a/packages/quicktype-core/src/language/JavaScript/JavaScriptRenderer.ts
+++ b/packages/quicktype-core/src/language/JavaScript/JavaScriptRenderer.ts
@@ -239,6 +239,16 @@ export class JavaScriptRenderer extends ConvenienceRenderer {
         };
     }
 
+    /** The expression a deserializer returns when runtime typechecks
+     * are disabled.  Subclasses can wrap it in a cast to the target
+     * type if `parsedJson`'s type isn't assignable to it. */
+    protected uncheckedParsedJson(
+        _t: Type,
+        parsedJson: Sourcelike,
+    ): Sourcelike {
+        return parsedJson;
+    }
+
     protected emitConvertModuleBody(): void {
         const converter = (t: Type, name: Name): void => {
             const typeMap = this.typeMapTypeFor(t);
@@ -251,7 +261,11 @@ export class JavaScriptRenderer extends ConvenienceRenderer {
                             ? "JSON.parse(json)"
                             : "json";
                     if (!this._jsOptions.runtimeTypecheck) {
-                        this.emitLine("return ", parsedJson, ";");
+                        this.emitLine(
+                            "return ",
+                            this.uncheckedParsedJson(t, parsedJson),
+                            ";",
+                        );
                     } else {
                         this.emitLine(
                             "return cast(",
diff --git a/packages/quicktype-core/src/language/TypeScriptFlow/FlowRenderer.ts b/packages/quicktype-core/src/language/TypeScriptFlow/FlowRenderer.ts
index a0a3bfaee2..9e48bfb147 100644
--- a/packages/quicktype-core/src/language/TypeScriptFlow/FlowRenderer.ts
+++ b/packages/quicktype-core/src/language/TypeScriptFlow/FlowRenderer.ts
@@ -1,17 +1,42 @@
 import type { Name } from "../../Naming.js";
+import type { Sourcelike } from "../../Source.js";
 import { utf16StringEscape } from "../../support/Strings.js";
 import { defined } from "../../support/Support.js";
-import type { ClassType, EnumType } from "../../Type/index.js";
+import type { ClassType, EnumType, Type } from "../../Type/index.js";
 import type { JavaScriptTypeAnnotations } from "../JavaScript/index.js";
 
 import { TypeScriptFlowBaseRenderer } from "./TypeScriptFlowBaseRenderer.js";
 import { tsFlowTypeAnnotations } from "./utils.js";
 
 export class FlowRenderer extends TypeScriptFlowBaseRenderer {
+    protected anyType(): string {
+        return this._tsFlowOptions.preferUnknown ? "mixed" : "any";
+    }
+
     protected forbiddenNamesForGlobalNamespace(): string[] {
         return ["Class", "Date", "Object", "String", "Array", "JSON", "Error"];
     }
 
+    protected uncheckedParsedJson(t: Type, parsedJson: Sourcelike): Sourcelike {
+        // With `raw-type any` and `prefer-unknown` the deserializer's
+        // parameter is `mixed`, which can't be returned as the target
+        // type without a cast.
+        if (
+            this._tsFlowOptions.rawType !== "json" &&
+            this._tsFlowOptions.preferUnknown
+        ) {
+            return [
+                "((",
+                parsedJson,
+                ": any): ",
+                this.sourceFor(t).source,
+                ")",
+            ];
+        }
+
+        return parsedJson;
+    }
+
     protected get typeAnnotations(): JavaScriptTypeAnnotations {
         return { never: "", ...tsFlowTypeAnnotations };
     }
diff --git a/packages/quicktype-core/src/language/TypeScriptFlow/TypeScriptFlowBaseRenderer.ts b/packages/quicktype-core/src/language/TypeScriptFlow/TypeScriptFlowBaseRenderer.ts
index d20f56c19c..d833317d86 100644
--- a/packages/quicktype-core/src/language/TypeScriptFlow/TypeScriptFlowBaseRenderer.ts
+++ b/packages/quicktype-core/src/language/TypeScriptFlow/TypeScriptFlowBaseRenderer.ts
@@ -64,7 +64,7 @@ export abstract class TypeScriptFlowBaseRenderer extends JavaScriptRenderer {
 
         return matchType<MultiWord>(
             t,
-            (_anyType) => singleWord("any"),
+            (_anyType) => singleWord(this.anyType()),
             (_nullType) => singleWord("null"),
             (_boolType) => singleWord("boolean"),
             (_integerType) => singleWord("number"),
@@ -113,6 +113,12 @@ export abstract class TypeScriptFlowBaseRenderer extends JavaScriptRenderer {
         );
     }
 
+    /** The type emitted for `any`-typed values in type declarations
+     * and converter signatures: the language's type-safe top type
+     * (`unknown` for TypeScript, `mixed` for Flow) with the
+     * `prefer-unknown` option, plain `any` without it. */
+    protected abstract anyType(): string;
+
     protected abstract emitEnum(e: EnumType, enumName: Name): void;
 
     protected abstract emitClassBlock(c: ClassType, className: Name): void;
@@ -198,7 +204,7 @@ export abstract class TypeScriptFlowBaseRenderer extends JavaScriptRenderer {
 
     protected deserializerFunctionLine(t: Type, name: Name): Sourcelike {
         const jsonType =
-            this._tsFlowOptions.rawType === "json" ? "string" : "any";
+            this._tsFlowOptions.rawType === "json" ? "string" : this.anyType();
         return [
             "function to",
             name,
@@ -212,7 +218,7 @@ export abstract class TypeScriptFlowBaseRenderer extends JavaScriptRenderer {
     protected serializerFunctionLine(t: Type, name: Name): Sourcelike {
         const camelCaseName = modifySource(camelCase, name);
         const returnType =
-            this._tsFlowOptions.rawType === "json" ? "string" : "any";
+            this._tsFlowOptions.rawType === "json" ? "string" : this.anyType();
         return [
             "function ",
             camelCaseName,
@@ -227,6 +233,10 @@ export abstract class TypeScriptFlowBaseRenderer extends JavaScriptRenderer {
         return undefined;
     }
 
+    // The runtime typecheck helpers are deliberately dynamic, so they
+    // stay on `any` even with `prefer-unknown`.  Only the public API
+    // surface (type declarations and converter signatures) uses
+    // `anyType()`.
     protected get castFunctionLines(): [string, string] {
         return [
             "function cast<T>(val: any, typ: any): T",
diff --git a/packages/quicktype-core/src/language/TypeScriptFlow/TypeScriptRenderer.ts b/packages/quicktype-core/src/language/TypeScriptFlow/TypeScriptRenderer.ts
index 376036727d..1d0d488424 100644
--- a/packages/quicktype-core/src/language/TypeScriptFlow/TypeScriptRenderer.ts
+++ b/packages/quicktype-core/src/language/TypeScriptFlow/TypeScriptRenderer.ts
@@ -9,13 +9,31 @@ import { TypeScriptFlowBaseRenderer } from "./TypeScriptFlowBaseRenderer.js";
 import { tsFlowTypeAnnotations } from "./utils.js";
 
 export class TypeScriptRenderer extends TypeScriptFlowBaseRenderer {
+    protected anyType(): string {
+        return this._tsFlowOptions.preferUnknown ? "unknown" : "any";
+    }
+
     protected forbiddenNamesForGlobalNamespace(): string[] {
         return ["Array", "Date"];
     }
 
+    protected uncheckedParsedJson(t: Type, parsedJson: Sourcelike): Sourcelike {
+        // With `raw-type any` and `prefer-unknown` the deserializer's
+        // parameter is `unknown`, which can't be returned as the target
+        // type without a cast.
+        if (
+            this._tsFlowOptions.rawType !== "json" &&
+            this._tsFlowOptions.preferUnknown
+        ) {
+            return [parsedJson, " as ", this.sourceFor(t).source];
+        }
+
+        return parsedJson;
+    }
+
     protected deserializerFunctionLine(t: Type, name: Name): Sourcelike {
         const jsonType =
-            this._tsFlowOptions.rawType === "json" ? "string" : "any";
+            this._tsFlowOptions.rawType === "json" ? "string" : this.anyType();
         return [
             "public static to",
             name,
@@ -29,7 +47,7 @@ export class TypeScriptRenderer extends TypeScriptFlowBaseRenderer {
     protected serializerFunctionLine(t: Type, name: Name): Sourcelike {
         const camelCaseName = modifySource(camelCase, name);
         const returnType =
-            this._tsFlowOptions.rawType === "json" ? "string" : "any";
+            this._tsFlowOptions.rawType === "json" ? "string" : this.anyType();
         return [
             "public static ",
             camelCaseName,
diff --git a/packages/quicktype-core/src/language/TypeScriptFlow/language.ts b/packages/quicktype-core/src/language/TypeScriptFlow/language.ts
index d7f0f208ba..a894ee2833 100644
--- a/packages/quicktype-core/src/language/TypeScriptFlow/language.ts
+++ b/packages/quicktype-core/src/language/TypeScriptFlow/language.ts
@@ -45,6 +45,11 @@ export const tsFlowOptions = {
         false,
     ),
     readonly: new BooleanOption("readonly", "Use readonly type members", false),
+    preferUnknown: new BooleanOption(
+        "prefer-unknown",
+        "Use unknown (TypeScript) or mixed (Flow) instead of any",
+        true,
+    ),
 };
 
 export const typeScriptLanguageConfig = {
diff --git a/test/languages.ts b/test/languages.ts
index 94f9828e3a..07f9a417a1 100644
--- a/test/languages.ts
+++ b/test/languages.ts
@@ -864,6 +864,7 @@ export const TypeScriptLanguage: Language = {
         { "acronym-style": "pascal" },
         { converters: "all-objects" },
         { readonly: "true" },
+        { "prefer-unknown": "false" },
     ],
     sourceFiles: ["src/language/TypeScript/index.ts"],
 };
@@ -943,6 +944,7 @@ export const FlowLanguage: Language = {
         { "runtime-typecheck": "false" },
         { "runtime-typecheck-ignore-unknown-properties": "true" },
         { "nice-property-names": "true" },
+        { "prefer-unknown": "false" },
     ],
     sourceFiles: ["src/language/Flow/index.ts"],
 };