The move means the UK bank can make even stronger its position as the biggest market maker on the index.
The deal will also reduce the number of designated market makers on the NYSE to just four with electronic systems becoming more widely used.
Market makers help improve liquidity by maintaining buy and sell quotes on all the companies they cover.
LaBranche is responsible for about 700 securities, making it the third-largest designated market maker by number of securities traded.
William White, the head of Barclays Capital Market Makers, said: "This transaction underscores our commitment to our DMM business and to promoting competitive and transparent public markets.
"We look forward to continuing our partnership with the New York Stock Exchange in offering our market-leading equities capabilities to an expanded client base."