The 27-year-old had already been on loan with the Hatters during the 2014-15 season from West Ham when John Still was in charge, before being signed permanently by boss Nathan Jones two years later in the summer of 2017.
He scored 12 goals as Town won promotion from League Two during first full campaign, a tally he repeated as Luton were crowned League One champions the following year.
Lee fell out of favour when Graeme Jones was appointed manager, but the return of Nathan Jones in May 2020 saw the former Hammer back in the side, netting a crucial goal in the 2-0 win at Huddersfield as the Hatters avoided relegation from the Championship on the final day.
The following two seasons saw his game time at Kenilworth Road limited, with a stint on loan at Oxford United last term, spending the majority of the current campaign with League One Charlton Athletic.
Lee departs the Hatters having made 124 appearances in total, netting 30 goals.