BUENOS AIRES—Argentina’s government imposed capital controls in its latest bid to prevent a depletion of foreign-currency reserves amid a crisis of confidence that has fueled dollar outflows and financial-market turbulence.

The unexpected measure, which takes effect on Monday, comes after President Mauricio Macri’s government sought last week to unilaterally extend the maturity of all short-term paper after it was unable to roll over obligations as a result of plunging demand for government debt.

“It’s...