diff --git a/dvc/repo/experiments/__init__.py b/dvc/repo/experiments/__init__.py
index e3fefc605..30b9a5d55 100644
--- a/dvc/repo/experiments/__init__.py
+++ b/dvc/repo/experiments/__init__.py
@@ -10,6 +10,7 @@ from typing import Dict, Iterable, Mapping, Optional
 
 from funcy import cached_property, first
 
+from dvc.dependency.param import MissingParamsError
 from dvc.env import DVCLIVE_RESUME
 from dvc.exceptions import DvcException
 from dvc.path_info import PathInfo
@@ -350,9 +351,19 @@ class Experiments:
         )
         self.scm.add(self.args_file)
 
+    def _format_new_params_msg(self, new_params, config_path):
+        """Format an error message for when new parameters are identified"""
+        new_param_count = len(new_params)
+        pluralise = "s are" if new_param_count > 1 else " is"
+        param_list = ", ".join(new_params)
+        return (
+            f"{new_param_count} parameter{pluralise} missing "
+            f"from '{config_path}': {param_list}"
+        )
+
     def _update_params(self, params: dict):
         """Update experiment params files with the specified values."""
-        from dvc.utils.collections import merge_params
+        from dvc.utils.collections import NewParamsFound, merge_params
         from dvc.utils.serialize import MODIFIERS
 
         logger.debug("Using experiment params '%s'", params)
@@ -362,7 +373,11 @@ class Experiments:
             suffix = path.suffix.lower()
             modify_data = MODIFIERS[suffix]
             with modify_data(path, fs=self.repo.fs) as data:
-                merge_params(data, params[params_fname])
+                try:
+                    merge_params(data, params[params_fname], allow_new=False)
+                except NewParamsFound as e:
+                    msg = self._format_new_params_msg(e.new_params, path)
+                    raise MissingParamsError(msg)
 
         # Force params file changes to be staged in git
         # Otherwise in certain situations the changes to params file may be
diff --git a/dvc/utils/collections.py b/dvc/utils/collections.py
index f05cdb8f5..1b62cf743 100644
--- a/dvc/utils/collections.py
+++ b/dvc/utils/collections.py
@@ -6,6 +6,16 @@ from typing import Callable, Dict, Iterable, List, TypeVar, Union
 
 from pygtrie import StringTrie as _StringTrie
 
+from dvc.exceptions import DvcException
+
+
+class NewParamsFound(DvcException):
+    """Thrown if new params were found during merge_params"""
+
+    def __init__(self, new_params: List, *args):
+        self.new_params = new_params
+        super().__init__("New params found during merge", *args)
+
 
 class PathStringTrie(_StringTrie):
     """Trie based on platform-dependent separator for pathname components."""
@@ -80,11 +90,27 @@ def chunk_dict(d: Dict[_KT, _VT], size: int = 1) -> List[Dict[_KT, _VT]]:
     return [{key: d[key] for key in chunk} for chunk in chunks(size, d)]
 
 
-def merge_params(src: Dict, to_update: Dict) -> Dict:
-    """Recursively merges params with benedict's syntax support in-place."""
+def merge_params(src: Dict, to_update: Dict, allow_new: bool = True) -> Dict:
+    """
+    Recursively merges params with benedict's syntax support in-place.
+
+    Args:
+        src (dict): source dictionary of parameters
+        to_update (dict): dictionary of parameters to merge into src
+        allow_new (bool): if False, raises an error if new keys would be
+            added to src
+    """
     from ._benedict import benedict
 
     data = benedict(src)
+
+    if not allow_new:
+        new_params = list(
+            set(to_update.keys()) - set(data.keypaths(indexes=True))
+        )
+        if new_params:
+            raise NewParamsFound(new_params)
+
     data.merge(to_update, overwrite=True)
     return src
 
diff --git a/tests/func/experiments/test_experiments.py b/tests/func/experiments/test_experiments.py
index a06cedf2e..cf94871f5 100644
--- a/tests/func/experiments/test_experiments.py
+++ b/tests/func/experiments/test_experiments.py
@@ -7,6 +7,7 @@ import pytest
 from funcy import first
 
 from dvc.dvcfile import PIPELINE_FILE
+from dvc.exceptions import DvcException
 from dvc.repo.experiments.utils import exp_refs_by_rev
 from dvc.utils.serialize import PythonFileCorruptedError
 from tests.func.test_repro_multistage import COPY_SCRIPT
@@ -118,10 +119,6 @@ def test_failed_exp(tmp_dir, scm, dvc, exp_stage, mocker, caplog):
             ["foo[1]=[baz, goo]"],
             "{foo: [bar: 1, [baz, goo]], goo: {bag: 3}, lorem: false}",
         ],
-        [
-            ["lorem.ipsum=3"],
-            "{foo: [bar: 1, baz: 2], goo: {bag: 3}, lorem: {ipsum: 3}}",
-        ],
     ],
 )
 def test_modify_params(tmp_dir, scm, dvc, mocker, changes, expected):
@@ -148,6 +145,28 @@ def test_modify_params(tmp_dir, scm, dvc, mocker, changes, expected):
         assert fobj.read().strip() == expected
 
 
+@pytest.mark.parametrize(
+    "changes",
+    [["lorem.ipsum=3"], ["foo[0].bazar=3"]],
+)
+def test_add_params(tmp_dir, scm, dvc, changes):
+    tmp_dir.gen("copy.py", COPY_SCRIPT)
+    tmp_dir.gen(
+        "params.yaml", "{foo: [bar: 1, baz: 2], goo: {bag: 3}, lorem: false}"
+    )
+    stage = dvc.run(
+        cmd="python copy.py params.yaml metrics.yaml",
+        metrics_no_cache=["metrics.yaml"],
+        params=["foo", "goo", "lorem"],
+        name="copy-file",
+    )
+    scm.add(["dvc.yaml", "dvc.lock", "copy.py", "params.yaml", "metrics.yaml"])
+    scm.commit("init")
+
+    with pytest.raises(DvcException):
+        dvc.experiments.run(stage.addressing, params=changes)
+
+
 @pytest.mark.parametrize("queue", [True, False])
 def test_apply(tmp_dir, scm, dvc, exp_stage, queue):
     from dvc.exceptions import InvalidArgumentError
@@ -387,7 +406,7 @@ def test_no_scm(tmp_dir):
 @pytest.mark.parametrize("workspace", [True, False])
 def test_untracked(tmp_dir, scm, dvc, caplog, workspace):
     tmp_dir.gen("copy.py", COPY_SCRIPT)
-    tmp_dir.gen("params.yaml", "foo: 1")
+    tmp_dir.scm_gen("params.yaml", "foo: 1", commit="track params")
     stage = dvc.run(
         cmd="python copy.py params.yaml metrics.yaml",
         metrics_no_cache=["metrics.yaml"],