The 32-year-old striker has agreed a deal at The Den until the end of the season and links up again with Lions manager Ian Holloway, who was his boss at Blackpool.
Campbell is Holloway's second January signing following the capture of young Tottenham full-back Ryan Fredericks last Thursday.
He will shortly fly out to Portugal and meet up with his new team-mates at their warm weather training camp.
Campbell, who has also played for Brentford, QPR, Leicester, Ipswich and Birmingham, could make his Lions debut against Sheffield Wednesday next Tuesday.
Rovers recently confirmed Campbell was one of six players arrested and later bailed without charge until April as part of a National Crime Agency investigation into spot-fixing in football.
- Sports & Recreation
- Ryan Fredericks