Tomlinson, 21, has been given the number 28 shirt by the Championship club and joins on a non-contract basis.
The pop star played in a charity match at the Keepmoat Stadium last season and the club say they hope he will appear for Paul Dickov's side at some stage of the season and raise money for the charity.
Doncaster say Tomlinson will remain a member of the band, who have become global sensations with No. 1 hits across the world following their creating during the 2010 edition of X-Factor.
Tomlinson said: “This has always been a childhood dream for me. I feel very honoured to have been asked to sign for Rovers and being able to help both the club and the amazing charity Bluebell Wood is what it’s all about.”
The star - part of the five-piece who have melted the hearts of young girls across the world - once worked at Rovers in his youth and though the signing is no doubt little more than a publicity stunt, it should be beneficial to Bluebell Wood.
Chairman John Ryan said: “I am honoured to let Louis play in my number to help raise money for the charity. I wore this in 2003 for my record beating appearance at Hereford and this is the first time I have allowed it to be used since but I felt this was a good cause.”