Williamson will cost Pompey around £3million, according to reports.
Pompey boss Paul Hart told the club’s official website: “He’s a young, hungry centre-half, a big lad who’s uncompromising and does the basics first.
“We’ve watched him play and he’s shown great promise which hopefully he will build on under our guidance.
“We were a centre-half short before Sylvain Distin left but now we have Tal Ben-Haim and Mike Williamson.
“We signed Ben-Haim to fill a role and Mike has come here to prove himself as a front-line player.”