Rut Was a Convert
Rut 4:11 Then all the people who were at the gate, and the elders, said, "[We are] witnesses! May Hashem make the woman who is coming into your house like Rachel and like Leah, but of whom built up the House of Yisrael. May you prosper in Efrat and be famous in Beit-Lechem; 12 and may your house be like the house of Peretz who Tamar bore to Yehudah, through the offspring which Hashem will give you by this young woman.
Since we have here stated that the 'elders' witnessed and approved this marriage and expected that Hashem would favor the marriage it would be ridiculous to not see that Rut had to have converted to Judaism before marrying Boaz. If Boaz was transgressing the Torah mitzvah of Devarim 7:3 by marrying a Gentile woman they would not have blessed the marriage. Nor would they have blessed Rut to become as great as the Matriarchs Rachel and Leah.