diff --git a/index.js b/index.js
index cf73e7c..d1fa1ce 100644
--- a/index.js
+++ b/index.js
@@ -58,8 +58,24 @@ function getMainArgs() {
   return ArrayPrototypeSlice(process.argv, 2);
 }
 
-function storeOptionValue(options, longOption, value, result) {
-  const optionConfig = options[longOption] || {};
+function storeOptionValue(strict, options, longOption, value, result) {
+  const hasOptionConfig = ObjectHasOwn(options, longOption);
+
+  if (strict) {
+    if (!hasOptionConfig) {
+      throw new Error(`Unknown option: --${longOption}`);
+    }
+
+    if (options[longOption].type === 'string' && value == null) {
+      throw new Error(`Missing value for 'string' option: --${longOption}`);
+    }
+
+    if (options[longOption].type === 'boolean' && value != null) {
+      throw new Error(`Unexpected value for 'boolean' option: --${longOption}`);
+    }
+  }
+
+  const optionConfig = hasOptionConfig ? options[longOption] : {};
 
   // Flags
   result.flags[longOption] = true;
@@ -83,9 +99,11 @@ function storeOptionValue(options, longOption, value, result) {
 
 const parseArgs = ({
   args = getMainArgs(),
+  strict = false,
   options = {}
 } = {}) => {
   validateArray(args, 'args');
+  validateBoolean(strict, 'strict');
   validateObject(options, 'options');
   ArrayPrototypeForEach(
     ObjectEntries(options),
@@ -166,6 +184,7 @@ const parseArgs = ({
         // - preserve information for author to process further
         const index = StringPrototypeIndexOf(arg, '=');
         storeOptionValue(
+          strict,
           options,
           StringPrototypeSlice(arg, 0, index),
           StringPrototypeSlice(arg, index + 1),
@@ -183,12 +202,12 @@ const parseArgs = ({
         const val = options[arg] && options[arg].type === 'string' ?
           args[++pos] :
           undefined;
-        storeOptionValue(options, arg, val, result);
+        storeOptionValue(strict, options, arg, val, result);
       } else {
         // Cases when an arg is specified without a value, example
         // '--foo --bar' <- 'foo' and 'bar' flags should be set to true and
         // save value as undefined
-        storeOptionValue(options, arg, undefined, result);
+        storeOptionValue(strict, options, arg, undefined, result);
       }
     } else {
       // Arguments without a dash prefix are considered "positional"
diff --git a/test/index.js b/test/index.js
index a393bf8..e089575 100644
--- a/test/index.js
+++ b/test/index.js
@@ -381,6 +381,76 @@ test('invalid union value passed to "type" option', function(t) {
   t.end();
 });
 
+// Test strict mode
+
+test('unknown long option --bar', function(t) {
+  const passedArgs = ['--foo', '--bar'];
+  const passedOptions = { foo: { type: 'boolean' } };
+  const strict = true;
+
+  t.throws(function() { parseArgs({ strict, args: passedArgs, options: passedOptions }); }, {
+    code: 'ERR_UNKNOWN_OPTION'
+  });
+
+  t.end();
+});
+
+test('unknown short option -b', function(t) {
+  const passedArgs = ['--foo', '-b'];
+  const passedOptions = { foo: { type: 'boolean' } };
+  const strict = true;
+
+  t.throws(function() { parseArgs({ strict, args: passedArgs, options: passedOptions }); }, {
+    code: 'ERR_UNKNOWN_OPTION'
+  });
+
+  t.end();
+});
+
+test('unknown option -r in short option group -bar', function(t) {
+  const passedArgs = ['--foo', '-bar'];
+  const passedOptions = { foo: { type: 'boolean' }, b: { type: 'boolean' }, a: { type: 'boolean' } };
+  const strict = true;
+
+  t.throws(function() { parseArgs({ strict, args: passedArgs, options: passedOptions }); }, {
+    code: 'ERR_UNKNOWN_OPTION'
+  });
+
+  t.end();
+});
+
+test('unknown option with explicit value', function(t) {
+  const passedArgs = ['--foo', '--bar=baz'];
+  const passedOptions = { foo: { type: 'boolean' } };
+  const strict = true;
+
+  t.throws(function() { parseArgs({ strict, args: passedArgs, options: passedOptions }); }, {
+    code: 'ERR_UNKNOWN_OPTION'
+  });
+
+  t.end();
+});
+
+test('string option used as boolean', function(t) {
+  const passedArgs = ['--foo'];
+  const passedOptions = { foo: { type: 'string' } };
+  const strict = true;
+
+  t.throws(function() { parseArgs({ strict, args: passedArgs, options: passedOptions }); });
+
+  t.end();
+});
+
+test('boolean option used with value', function(t) {
+  const passedArgs = ['--foo=bar'];
+  const passedOptions = { foo: { type: 'boolean' } };
+  const strict = true;
+
+  t.throws(function() { parseArgs({ strict, args: passedArgs, options: passedOptions }); });
+
+  t.end();
+});
+
 test('invalid short option length', function(t) {
   const passedArgs = [];
   const passedOptions = { foo: { short: 'fo' } };