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.
9. Sean Davis - 116 appearances
Played an important part of the ‘Great Escape’ in 2006, and proved to be an integral cog in Harry Redknapp’s midfield machine - although, most famously, didn't make the squad for the 2008 FA Cup final win against Cardiff. Photo: Barry Coombs
10. Kyle Bennett - 117 appearances
After impressing for Doncaster Rovers, Bennett became Paul Cook’s first Pompey signing. Made his 100th appearance in August 2017, and scored a total of 13 goals and registered 21 assists for the club. Photo: Joe Pepler
11. Lee Brown - 117 appearances
In rich goal scoring form at present, Brown has been a more or less ever-present for Pompey since his arrival in 2018. Due to Clark Robertson’s injury, the 31-year-old is currently captaining Cowley’s side. Photo: Jason Brown
12. Arjan de Zeeuw - 118 appearances
Joined on a free transfer from Wigan and quickly became a key member of the Blues squad. After gaining promotion to the Premier League in 2003, the Dutch defender won the Fans’ Player of the Year the following year. Photo: PICTURE:STEVE REID(042144-9)