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.
13. Jamal Lowe - 119 appearances
Lowe grew to prominence in non-league football before Pompey signed the attacker in 2017. Scored a memorable goal in the EFL Trophy final against Sunderland before joining former manager Cook at Wigan. Has since played for Swansea and is now at Bournemouth. Photo: Daniel Chesterton
14. Jed Wallace - 121 appearances
A graduate of Pompey’s academy, Wallace became a fans’ favourite during a difficult period for the club. Scored a total of 30 goals before departing for Wolves in 2015 and has since established himself in the Championship with Millwall. Photo: Joe Pepler
15. Conor Chaplin - 122 appearances
Another graduate of the Blues' academy and a huge Fratton Park favourite. Chaplin won promotion to League One with the club in 2016-17 and the Fratton faithful would love to see him back on day. Photo: Joe Pepler
16. Hermann Hreidarsson - 123 appearances
The former Iceland international became one of Pompey’s most beloved players during his five-year stay on the south coast. Hreidarsson lifted the FA Cup in 2008 and scored eight goals for the club. Photo: Steve Reid