Winger Etuhu spent most of the 2011/12 season at Portsmouth and was expected to stay at the south-coast club despite their relegation to League One.
However, the 24-year-old has now penned a one-year deal with the Tykes, much to the delight of boss Keith Hill.
He told the club's official website: "We're glad we've managed to secure the signing of Kelvin. We're excited by the prospect of working with him.
"We're really pleased and he represents good value for money and I'm just glad he chose us.
"It's exciting because he can play in a number of positions - anywhere across the front line. He's got pace and ability and he will excite the adoring public."