The 31-year-old, who left Derby earlier this month after his contract at Pride Park was cancelled by mutual consent, featured for a Lions reserve side at Gillingham on Tuesday.
Riggott's career has been marred by a series of injuries that have seen him make just ten competitive first-team appearances since 2008.
The former Middlesbrough, Stoke and Cardiff centre-back played all 90 minutes at Priestfield, where Dany N'Guessan's second-half header sealed a narrow win for the visitors.
Fellow trialist Marin Pusic, the ex-Hull winger, was also involved.
Millwall: Allsop, Hackett, Riggott, De Havilland, McClaren, Batt (Sammoutis 60), Henry, Wright, Marquis, N'Guessan, Pusic.