The yuan’s rapid slide is posing a new test for Beijing as it grapples with a looming trade war, putting the central bank under pressure to revive the kind of interventions undertaken in the past to defend the currency’s value.

In an effort to calm jittery investors, China’s central-bank chief Yi Gang on Tuesday pledged to keep the yuan’s exchange rate “basically stable at a reasonable and balanced level,” according to an article posted on the website of the People’s Bank of China. The currency, which had fallen as much as...