Dennis is now searching for a new club after spending two years at Brisbane Road.
Pompey plucked the ex-Dagenham & Redbridge man from non-league Bromley in 2018, having scored 21 goals and provided 12 assists in 53 appearances during the previous season.
However, Dennis struggled to make any sort of impact during his year-long spell at Fratton Park.
He played eight times, netting one goal and made only a solitary League One appearance off the bench at Oxford United.
And despite starting the Blues' first friendly of the summer in 2019 against UCD where he recorded two goals and two assists, Dennis was subsequently sold to Orient for a nominal fee.
However, Dennis didn't hit the heights some might have expected for the League Two outfit.
In his maiden campaign at the Londoners, he scored twice in 21 matches before netting three goals in 28 games this term.
As a result, the 28-year-old has not earned fresh terms at Brisbane Road and is again a free agent.
Writing on Twitter, Dennis said: ‘The end of another chapter. I would like to thank @leytonorientfc for the last 2 seasons & thank the fans who have supported us through a really tough season due to all of the circumstances. Best wishes for the future.’