An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration of the object relative to an observer who is in free fall. Proper acceleration is different from coordinate acceleration, which is acceleration with respect to a given coordinate system, which may or may not be accelerating. For example, an accelerometer at rest on the surface of the Earth will measure an acceleration due to Earth's gravity straight upwards of about g ≈ 9.81 m/s². By contrast, an accelerometer that is in free fall will measure zero acceleration.

कोई भी ऐसी युक्ति जो त्वरण मापने के काम आती है, त्वरणमापी (accelerometer) कहलाती है। प्रायः ये विद्युतयांत्रिक (एलेक्ट्रोमेकैनिकल) युक्तियाँ होती हैं जो त्वरण के संगत उपयुक्त वैद्युत संकेत प्रदान करती हैं।