Halford's Cardiff deal expired at the end of last month, although he has continued to train with the Bluebirds since then.
The 33-year-old confirmed on Twitter, however, that today (Friday) is his final day at the club, although his next destination remains to be seen at this stage.
After his farewell message to Cardiff on Thursday, Halford was inundated with messages from Portsmouth fans looking to gauge his interest in signing for their side.
And in his only reply so far, the Essex-born player said:
Think everyone knows I'd come back if the opportunity comes up.— Greg Halford (@GregHalford15) 12 July 2018
Halford, who has played for 12 different clubs throughout his career, spent the best part of two seasons at Portsmouth - scoring or assisting 17 goals in 77 games - but was unable to prevent Pompey's relegation to the Championship in the last of those.
The defender or midfielder also played briefly under Portsmouth manager Kenny Jackett at Rotherham United.