Following speculation that he would sign in the January transfer window, Boyd officially joined Rangers on 1 January 2006. He was signed for a fee of around £500,000 on a contract lasting until the summer of 2009. He waived half of his £40,000 signing on fee, which Kilmarnock were due to pay him under the terms of his contract, to help fund the youth setup at Kilmarnock.
He made his debut on 7 January 2006 against Peterhead in the Scottish Cup third round, in which he scored a hat-trick during a 5–0 win. He went on to score 20 goals in 17 starts for Rangers in the second half of that season, ending it with a total of 37 goals for Rangers and Kilmarnock. He became the first player to finish top scorer at two clubs in one season, having scored 17 goals for Kilmarnock before his move.
On 11 May 2006, he made his debut for the Scotland national team and scored twice in a 5–1 win over Bulgaria in the Kirin Cup. Boyd then took part in European Championship qualifying, scoring twice against the Faroe Islands in a 6–0 win and also against Georgia in a 2–1 win, his fifth goal in six games.