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.
5. Sean Raggett - 101 appearances
The no-nonsense centre-back is establishing himself as a real fans' favourite at Fratton Park following his initial loan move Norwich in 2019. Photo: Paul Thompson
6. Sol Campbell - 111 appearances
Campbell spent three years at Fratton Park after leaving Arsenal and captained Pompey to the FA Cup in 2008. Forged two formidable partnerships - firstly with Linvoy Primus, then Sylvain Distin. Photo: Nick Potts
7. John Marquis - 114 appearances
A prominent figure for the Blues across the past three seasons, although Marquis has produced more questions than answers during that time. Has struggled to produce the form at Doncaster that prompted his signing. He's scored 37 goals in those 114 appearances over two spells. Photo: Jason Brown
8. John Utaka -114 appearances
A reported £7m signing from Rennes in 2007, Utaka scored 13 goals during his time at Fratton Park. Another member of the squad that won the FA Cup, he eventually left for Montpellier in 2011. Photo: Allan Hutchings