NEW YORK (Reuters) - New York Mayor Michael Bloomberg, who engineered a change in the city's term-limits law so he could run again and set a U.S. record for financing his campaign, won a third term on Tuesday, local media declared.

Bloomberg defeated City Comptroller Bill Thompson, a Democrat, according to NY1 television. With 91 percent of the votes counted, Bloomberg was ahead 50 to 47 percent.

His margin was far smaller than expected, given polls that showed him as recently as Monday with a double-digit lead.

Bloomberg, described by Forbes as the richest man in New York with a $16 billion fortune, dramatically outspent his challenger, laying out $13 for every $1 spent by Thompson.

Bloomberg has spent almost $90 million on his re-election bid and is on track to spend an estimated $110 million to $140 million overall. Thompson spent $7 million.

(Reporting and writing by Ellen Wulfhorst; Additional reporting by Edith Honan; Editing by Philip Barbara)