The 18 year old defender moves from his parent club Derby County.
"We're very pleased Lewin has agreed to come here and very grateful to Derby for letting us have him until the end of the season," said Barnsley manager Simon Davey.
"A lot of clubs have been in for him over the last 12 months and there's been talk of a big move.
"Derby are top of the table and have been good enough to lend him to us. It's a great move for everyone and he will fit in very well here.
"We felt central defence was an area we needed to strengthen and things moved forward after we watched him for Wales against Northern Ireland last week. He did very well."