On 4 September 2014 Guidetti joined Scottish Premiership side Celtic on loan for the 2014–15 season, but was ineligible for the UEFA Europe League group stages. He made his debut nine days later, replacing Scott Brown for the final 29 minutes of a 2–1 home win over Aberdeen. On 24 September he scored his first Celtic goal, opening a 3–0 home win over Heart of Midlothian in the last 16 of the Scottish League Cup. Three days later he scored his first Premiership goals, netting both as Celtic won 2–1 at St Mirren.
He scored a hat-trick on 29 October, as Celtic advanced to the semi-finals of the Scottish League Cup with a 6–0 win over Partick Thistle. It was the 15th hat-trick scored at Celtic by a Swedish player. Three days later he scored the only goal in a league win over Inverness Caledonian Thistle, his ninth goal in his last seven games. He was named as the Premiership Player of the Month for October 2014, with six goals in four games, all won by Celtic.