BEIJING—China’s central bank devalued its tightly controlled currency, causing its biggest one-day loss in two decades, as the world’s second-largest economy continues to sputter.

Chinese authorities said the change would help drive the currency toward more market-driven movements. The move also signaled the government’s growing worry about slow growth. A shift toward a weaker currency could help flagging exports at a time when many other efforts to boost the economy haven’t proven very effective.

China’s yuan has been on an upward track for a decade, during which the country’s economy grew to be the second largest in the world and the currency gained importance globally. The devaluation Tuesday was the most significant downward adjustment to the yuan since 1994, when as part of a break from Communist state planning, Beijing let the currency fall by one-third.

China sets a midpoint for the value of the yuan against the U.S. dollar. In daily trading, the yuan is allowed to move 2% above or below that midpoint, which is called the daily fixing. But the central bank sometimes ignores the daily moves, at times setting the fixing so that the yuan is stronger against the dollar a day after the market has indicated it should be weaker.

With Tuesday’s move, the fixing will now be based on how the yuan closes in the previous trading session. As a result, the yuan’s fixing was weakened by 1.9% Tuesday from the previous day, leaving it at 6.2298 to the U.S. dollar, compared with 6.1162 on Monday. The yuan dropped as much as 1.99% from its previous close to 6.3360 against the dollar in Shanghai and fell as much as 2.3% in Hong Kong in early trading.