Mark Baldwin left his home in Kenilworth Road at around 10.30am on Sunday (May 14) to go for a run.
But the 45-year-old did not return home and now Hampshire Constabulary, and his family, are very concerned for his welfare.
Mr Baldwin is white, around 5ft 9ins tall, of medium build with very short blond hair and blue eyes.
He was last seen wearing black running trousers with a little bit of blue on, a black jacket and blue trainers.
Police said Mr Baldwin usually goes for a run around Fleet Pond , along the Basingstoke Canal or in Velmead Woods but it is possible he chose a different route on Sunday.
A spokesman said: "If you’re out and about please keep a look out. If you think you’ve seen Mark, please call us as soon as possible.”
Anyone with information is asked to call Hampshire Constabulary on 101.