This looks like a service badge (ger. Dienstauszeichnung) of the Wehrmacht, issued since 1935 to all the servicemen who have spent a required number of years in the armed forces. i.e. 4. 12, 18, 25 and 40 years for a 4th, 3rd, 2nd, 1st and special class of the badge respectively (the last one having been introduced only in 1939). These were presented from their introduction to the Spring 1940 when the issuing has been postponed 'until the end of war' (effectively finished as Wehrmacht has been dissolved after the Germany's surrender). The obverse of the 4th and 3rd class had the Wehrmacht eagle with the swastika held in the talons, surrounded by the inscription 'Treue Dienste in der Wehrmacht' (Loyal service in the Armed Forces), and the reverse was presenting the number of service years within an wreath of oak leaves. The 2nd and 1st class badges had a form of a cross with a round shield in the middle, adorned with the eagle with a swastika (but without the inscription). Obverse would bear the number of years within the wreath, like all other badge types. The special badge for 40 years of service lacked the number, but had a gold cluster of oak leaves added to a ribbon. In addition, the 4th and 2nd class badges were mat silver, while 3rd, 1st and special class were gilded.

I am not an expert when it comes to the authenticity of the military decorations, but this particular specimen looks a bit suspicious. First and foremost, it should have an eyelet attached to the rim, so that it could have been fitted to a ribbon for wearing. On the other hand, lack of this element can be attributed to a casual damage. The other, more important thing is that this specimen seems to be 4th class badge, as evidenced by the number '4' on the obverse, but these had matt silver coating, not golden ones, like the 3rd class badges did. But the badges themselves were made from either from tombac or mild steel, so it is possible that this is a tombac 4th class badge that had its coating removed or not applied for some reason, but this should raise a suspicion and if you're interested in the authenticity of the badge, you should consult an expert.