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.
33. Gareth Evans - 218 appearances
Evans has made the second most Pompey appearances in the 21st century and was crucial to the club due to his versatility and strong attributes. He lifted the League Two title under Paul Cook and was appointed vice-captain by Kenny Jackett. He joined Bradford in 2020 after finding game time under Jackett increasingly tough. Photo: Joe Pepler
34. Linvoy Primus - 219 appearances
Primus grew alongside the club and continually adapted to new surroundings in the old Division One and then the Premier League. He scored his first Premier League goal against Crystal Palace and played a key role in the club establishing itself in the top flight. Photo: Chris Ison