...there is an amazing Midrash [Brought in[Cite by the Rif on Ein Yaakov Sanhedrin 107a, Me’am Lo’ez on Shmuel Beisbeis perek 11]11 and the Alshich on Shmuel beis perek 13] which reveals that Uriyah was the arms-bearer of Goliath. After Dovid struck Goliath down with his Divinely-guided slingshot, Dovid could not draw Goliath’s sword to chop off his head. Amazingly, Uriyah, Goliath’s arms-bearer, helped Dovid finish off the job.
It would appear from here that Uriah was at one point not Jewish. Based on the other answer it would seem he converted.