diff --git a/packages/vitest/src/utils/config-helpers.ts b/packages/vitest/src/utils/config-helpers.ts
index e170b882de0a..6d63e5a4336e 100644
--- a/packages/vitest/src/utils/config-helpers.ts
+++ b/packages/vitest/src/utils/config-helpers.ts
@@ -23,14 +23,20 @@ export function getOutputFile(config: PotentialConfig | undefined, reporter: Bui
  */
 export function wrapSerializableConfig(config: ResolvedConfig) {
   let testNamePattern = config.testNamePattern
+  let defines = config.defines
 
   // v8 serialize does not support regex
   if (testNamePattern && typeof testNamePattern !== 'string')
     testNamePattern = `${REGEXP_WRAP_PREFIX}${testNamePattern.toString()}` as unknown as RegExp
 
+  // v8 serialize drops properties with undefined value
+  if (defines)
+    defines = wrapDefines(defines)
+
   return {
     ...config,
     testNamePattern,
+    defines,
   } as ResolvedConfig
 }
 
@@ -45,5 +51,20 @@ export function unwrapSerializableConfig(config: ResolvedConfig) {
       config.testNamePattern = parseRegexp(testNamePattern.slice(REGEXP_WRAP_PREFIX.length))
   }
 
+  if (config.defines && Array.isArray(config.defines.keys)) {
+    const { keys, original } = config.defines as ReturnType<typeof wrapDefines>
+    const defines: ResolvedConfig['defines'] = {}
+
+    // Apply all keys from the original. Entries which had undefined value are missing from original now
+    for (const key of keys)
+      defines[key] = original[key]
+
+    config.defines = defines
+  }
+
   return config
 }
+
+function wrapDefines(input: Record<string, unknown>) {
+  return { keys: Object.keys(input), original: input }
+}