The former Burton Albion attacker became a centurion in the 4-0 victory over Sunderland, and even netted in the 4-0 win.
He now joins current team-mates Ronan Curtis, John Marquis, and Lee Brown in reaching the milestone.
Click through to see who else Harness joins in Pompey’s 21st century 100 club.
17. Jamie Ashdown - 123 appearances
Ashdown spent eight years at Pompey and after biding his time became the Blues' number one in 2010. Perhaps his proudest achievement at Fratton Park was breaking a post-war club record for most minutes without conceding a goal between February and March 2011 (636 minutes). Photo: Barry Zee
18. Dejan Stefanovic - 125 appearances
Signed for the club upon promotion to the Premier League and established himself across the back four. Captained the club under Alain Perrin, before falling out with the Frenchman. He left Pompey in 2007, joining Fulham. Photo: Tony O'Brien
19. Hayden Mullins - 128 appearances
Mullins joined the club from West Ham in 2009 and started the 2010 FA Cup final defeat to Chelsea. He stayed at Fratton Park despite the Blues being relegated from the Premier League. Photo: Allan Hutchings
20. Craig MacGillivray - 135 appearances
MacGillivray won The News/Sports Mail Player of the Year award at the end of last season, before departing for Charlton in the summer. Saved a crucial penalty in the 2019 EFL Trophy shootout victory over Sunderland. Photo: Bryn Lennon