TOKYO, Jan 5 (Reuters) - The Chinese yuan posted its biggest percentage gains in about a year in offshore trading on Wednesday, after Chinese authorities stepped into both its onshore and offshore yuan markets to shore up the faltering yuan for a second day.

The yuan rose 1.3 percent by New York close on Wednesday to 6.8689 to the dollar, its biggest since Jan 11 last year, when it rose 1.5 percent.

(Reporting by Hideyuki Sano; Editing by Eric Meijer)