diff --git a/baseline-error-prone/src/main/java/com/palantir/baseline/errorprone/DefaultLocale.java b/baseline-error-prone/src/main/java/com/palantir/baseline/errorprone/DefaultLocale.java
new file mode 100644
index 000000000..966818425
--- /dev/null
+++ b/baseline-error-prone/src/main/java/com/palantir/baseline/errorprone/DefaultLocale.java
@@ -0,0 +1,64 @@
+/*
+ * (c) Copyright 2019 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.palantir.baseline.errorprone;
+
+import com.google.auto.service.AutoService;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.BugPattern.SeverityLevel;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.google.errorprone.fixes.SuggestedFixes;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.google.errorprone.matchers.method.MethodMatchers;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.MethodInvocationTree;
+
+@AutoService(BugChecker.class)
+@BugPattern(
+        link = "https://github.com/palantir/gradle-baseline#baseline-error-prone-checks",
+        linkType = BugPattern.LinkType.CUSTOM,
+        summary = "Implicit use of the platform default locale, which can result in differing behaviour between JVM"
+                + " executions.",
+        severity = SeverityLevel.SUGGESTION)
+public final class DefaultLocale extends BugChecker implements BugChecker.MethodInvocationTreeMatcher {
+
+    private static final long serialVersionUID = 1L;
+
+    private static final Matcher<ExpressionTree> MATCHER = MethodMatchers.instanceMethod()
+            .onExactClass("java.lang.String")
+            .namedAnyOf("toLowerCase", "toUpperCase")
+            .withNoParameters();
+
+    @Override
+    public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState state) {
+        if (MATCHER.matches(tree, state)) {
+            SuggestedFix.Builder fix = SuggestedFix.builder();
+            return buildDescription(tree)
+                    .addFix(fix.replace(
+                                    state.getEndPosition(tree.getMethodSelect()),
+                                    state.getEndPosition(tree),
+                                    String.format(
+                                            "(%s.ROOT)", SuggestedFixes.qualifyType(state, fix, "java.util.Locale")))
+                            .build())
+                    .build();
+        }
+
+        return Description.NO_MATCH;
+    }
+}
diff --git a/baseline-error-prone/src/test/java/com/palantir/baseline/errorprone/DefaultLocaleTest.java b/baseline-error-prone/src/test/java/com/palantir/baseline/errorprone/DefaultLocaleTest.java
new file mode 100644
index 000000000..9fc267211
--- /dev/null
+++ b/baseline-error-prone/src/test/java/com/palantir/baseline/errorprone/DefaultLocaleTest.java
@@ -0,0 +1,66 @@
+/*
+ * (c) Copyright 2020 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.palantir.baseline.errorprone;
+
+import org.junit.jupiter.api.Test;
+
+class DefaultLocaleTest {
+
+    @Test
+    void testFixToLowerCase() {
+        fix().addInputLines(
+                        "Test.java",
+                        "class Test {",
+                        "  String f(String s) {",
+                        "    return s.toLowerCase();",
+                        "  }",
+                        "}")
+                .addOutputLines(
+                        "Test.java",
+                        "import java.util.Locale;",
+                        "class Test {",
+                        "  String f(String s) {",
+                        "    return s.toLowerCase(Locale.ROOT);",
+                        "  }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    void testFixToUpperCase() {
+        fix().addInputLines(
+                        "Test.java",
+                        "class Test {",
+                        "  String f(String s) {",
+                        "    return s.toUpperCase();",
+                        "  }",
+                        "}")
+                .addOutputLines(
+                        "Test.java",
+                        "import java.util.Locale;",
+                        "class Test {",
+                        "  String f(String s) {",
+                        "    return s.toUpperCase(Locale.ROOT);",
+                        "  }",
+                        "}")
+                .doTest();
+    }
+
+    private RefactoringValidator fix() {
+        return RefactoringValidator.of(DefaultLocale.class, getClass());
+    }
+}
diff --git a/changelog/@unreleased/pr-2343.v2.yml b/changelog/@unreleased/pr-2343.v2.yml
new file mode 100644
index 000000000..e8f0785df
--- /dev/null
+++ b/changelog/@unreleased/pr-2343.v2.yml
@@ -0,0 +1,10 @@
+type: feature
+feature:
+  description: |-
+    Add DefaultLocale check
+
+    Related to https://github.com/google/error-prone/issues/632
+
+    Adds a `DefaultLocale` check that replaces uses of `String.toLowerCase()` and `String.toUpperCase()` with the overloads that take a `Locale`, using `Locale.ROOT`.
+  links:
+  - https://github.com/palantir/gradle-baseline/pull/2343