The 23-year-old forward has signed a three-year contract, becoming Pompey’s seventh summer addition.
Kamara returns after impressing during his 2023/24 loan spell, helping Portsmouth win the League One title.
Blues boss John Mousinho said: “We’re thrilled to have Abu back at Pompey – it’s no secret that we’ve wanted him to return for some time.
“I don’t need to tell you all about what he can offer the side, as you already saw it during that memorable season when he was here on loan.
“Abu returns a more experienced player, with Championship and top-flight Spanish appearances under his belt.”
The former England youth international scored 10 goals and provided 11 assists in 52 appearances. His standout moments included a brace against Shrewsbury as Pompey secured promotion.