in june 2004, sinclair signed for championship side burnley on a free transfer, signing a one-year contract. on the same day he went straight into the squad for the pre-season friendly against everton, starting at centre-half in the 1–1 draw. he made his debut for the club on 7 august in the opening match of the season against sheffield united. he struck up an impressive partnership at the centre of the defence with fellow new recruit john mcgreal at the start of the season, starting in the majority of games. on 24 december 2004, sinclair signed a new two-year contract extension until june 2007, with clarets manager steve cotterill stating that, "he has been an integral part of our defence". in january 2005, he was appointed as captain following the departure of robbie blake to birmingham city. on 5 february 2005, he received a red card in the local derby defeat to leeds united for receiving two bookings. his first goal for the club came on 12 march, in a 2–1 home win over rotherham united with sinclair scoring a header. towards the end of the season he received plaudits for his impressive partnership with young loan signing gary cahill and made a total of 42 appearances in his first season with the club as the side finished in mid-table.