The 27-year-old striker has put pen to paper on a two-year contract with the U’s and joins on a free transfer after Pompey decided against offering him a new deal.
“Ryan is another really strong addition to the squad,” said Appleton after making his third new signing.
“He works hard, scores goals, he leads the line really well and he is someone you know is going to make life very hard for opposition defenders. He was one of the best forwards in the division last season and we are delighted to have him here.”
Taylor follows Kemar Roofe and Liam Sercombe into the club this month.