diff --git a/datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeserializer.java b/datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeserializer.java
index ced787dd..c80bd5e6 100644
--- a/datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeserializer.java
+++ b/datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeserializer.java
@@ -424,9 +424,11 @@ protected T _fromString(JsonParser p, DeserializationContext ctxt,
             if (dots >= 0) { // negative if not simple number
                 try {
                     if (dots == 0) {
+                        _validateTimestampLength(p, string, false);
                         return _fromLong(ctxt, NumberInput.parseLong(string));
                     }
                     if (dots == 1) {
+                        _validateTimestampLength(p, string, true);
                         return _fromDecimal(ctxt, NumberInput.parseBigDecimal(string, false));
                     }
                 } catch (NumberFormatException e) {
diff --git a/datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/deser/JSR310DeserializerBase.java b/datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/deser/JSR310DeserializerBase.java
index 27b2f009..b48b9225 100644
--- a/datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/deser/JSR310DeserializerBase.java
+++ b/datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/deser/JSR310DeserializerBase.java
@@ -22,6 +22,8 @@
 
 import com.fasterxml.jackson.core.JsonParser;
 import com.fasterxml.jackson.core.JsonToken;
+import com.fasterxml.jackson.core.StreamReadConstraints;
+import com.fasterxml.jackson.core.exc.StreamConstraintsException;
 import com.fasterxml.jackson.core.io.NumberInput;
 import com.fasterxml.jackson.databind.DeserializationContext;
 import com.fasterxml.jackson.databind.JsonDeserializer;
@@ -238,4 +240,27 @@ protected DateTimeException _peelDTE(DateTimeException e) {
         }
         return e;
     }
+
+    /**
+     * Helper method to validate length of a stringified numeric Date/Time value
+     * against {@link StreamReadConstraints} limits.
+     *
+     * @param p Parser to get constraints from
+     * @param value Stringified numeric value to validate
+     * @param isFP Whether {@code value} is a floating-point (has decimal point)
+     *    or integer number; caller knows this already so we avoid re-scanning
+     *
+     * @since 2.18.10
+     */
+    protected void _validateTimestampLength(JsonParser p, String value, boolean isFP)
+        throws StreamConstraintsException
+    {
+        final int len = value.length();
+        StreamReadConstraints constraints = p.streamReadConstraints();
+        if (isFP) {
+            constraints.validateFPLength(len);
+        } else {
+            constraints.validateIntegerLength(len);
+        }
+    }
 }
diff --git a/datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/ModuleTestBase.java b/datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/ModuleTestBase.java
index cc6725a8..1ed01eb9 100644
--- a/datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/ModuleTestBase.java
+++ b/datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/ModuleTestBase.java
@@ -66,6 +66,14 @@ protected void verifyException(Throwable e, String... matches)
         throw new Error("Expected an exception with one of substrings ("+Arrays.asList(matches)+"): got one with message \""+msg+"\"");
     }
 
+    protected static String repeat(String s, int count) {
+        StringBuilder sb = new StringBuilder(s.length() * count);
+        for (int i = 0; i < count; i++) {
+            sb.append(s);
+        }
+        return sb.toString();
+    }
+
     protected static <T> Map<T, String> asMap(T key, String value) {
         return Collections.singletonMap(key, value);
     }
diff --git a/datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeserTest.java b/datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeserTest.java
index cc0f7d12..6222ac4f 100644
--- a/datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeserTest.java
+++ b/datetime/src/test/java/com/fasterxml/jackson/datatype/jsr310/deser/InstantDeserTest.java
@@ -9,6 +9,8 @@
 
 import org.junit.Test;
 
+import com.fasterxml.jackson.core.StreamReadConstraints;
+import com.fasterxml.jackson.core.exc.StreamConstraintsException;
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.exc.MismatchedInputException;
 
@@ -640,4 +642,40 @@ public void testISO8601ColonlessRegexDoesNotAffectNegativeYearsWithColonless() {
         assertTrue("Matcher finds +0100 as an colonless offset", matcher.find());
         assertEquals("Matcher groups +0100 as an colonless offset", matcher.group(), "+0100");
     }
+
+    // [modules-java8#387]: StreamReadConstraints should limit numeric string lengths
+    // parsed via _fromString to prevent excessive BigDecimal construction.
+    // NOTE: values MUST be quoted -- unquoted ones are Number tokens, limits for
+    // which are enforced by the streaming parser and not by this deserializer.
+    @Test
+    public void testNumericStringRespectsStreamReadConstraints() throws Exception
+    {
+        final int MAX_ALLOWED_LEN = StreamReadConstraints.DEFAULT_MAX_NUM_LEN;
+
+        // Normal epoch seconds as integer should work
+        assertNotNull(MAPPER.readValue(q("1234567890"), Instant.class));
+
+        // Normal epoch seconds with decimal should work
+        assertNotNull(MAPPER.readValue(q("1234567890.123456789"), Instant.class));
+
+        // A very long integer string (exceeding default 1000-digit limit) should fail
+        String longInt = repeat("1", MAX_ALLOWED_LEN + 1);
+        try {
+            MAPPER.readValue(q(longInt), Instant.class);
+            fail("Should not pass with excessively long integer string");
+        } catch (StreamConstraintsException e) {
+            verifyException(e, "Number value length");
+            verifyException(e, "exceeds the maximum allowed");
+        }
+
+        // A very long decimal string (exceeding default 1000-char limit) should fail
+        String longDecimal = "1234." + repeat("9", MAX_ALLOWED_LEN);
+        try {
+            MAPPER.readValue(q(longDecimal), Instant.class);
+            fail("Should not pass with excessively long decimal string");
+        } catch (StreamConstraintsException e) {
+            verifyException(e, "Number value length");
+            verifyException(e, "exceeds the maximum allowed");
+        }
+    }
 }
diff --git a/release-notes/VERSION-2.x b/release-notes/VERSION-2.x
index ac77f20b..1a71b206 100644
--- a/release-notes/VERSION-2.x
+++ b/release-notes/VERSION-2.x
@@ -8,6 +8,11 @@ Modules:
 === Releases ===
 ------------------------------------------------------------------------
 
+2.18.10 (not yet released)
+
+#387: Validate length of input in `InstantDeserializer`
+ (fix by @pjfanning)
+
 2.18.9 (07-Jul-2026)
 2.18.8 (28-May-2026)
 2.18.7 (24-Apr-2026)