RIYADH, Saudi Arabia—A Saudi court reduced the death sentences of five men convicted in the killing of dissident journalist Jamal Khashoggi to 20-year prison terms and handed shorter sentences to three other nationals, in a case that drove a wedge between Saudi Arabia’s heir-apparent and the West.

The public prosecutor declared the case closed nearly two years after Mr. Khashoggi was killed at the Saudi Consulate in Istanbul, undermining Crown Prince Mohammed bin Salman’s reputation among some of Saudi Arabia’s most important international partners and imperiling his economic reform program, despite his continued support from President Trump.

Those convicted had been expected to avoid execution after Mr. Khashoggi’s eldest son said in May his family had forgiven the killers, but the decision is unlikely to silence calls for accountability, including from influential members of Congress who have mulled a broad re-evaluation of the decades-old U.S.-Saudi alliance.

Prince Mohammed, the kingdom’s day-to-day ruler, has struggled to repair his battered image ahead of the summit of G-20 leaders, which Saudi Arabia is set to host in November. He hasn’t returned to the U.S. since the killing, following whirlwind visits to showcase his vision for Saudi Arabia and to pitch for investments.

Mr. Trump has stood by Prince Mohammed, despite the Central Intelligence Agency’s assessment that he likely ordered the killing. The prince has denied that, but said that as de facto leader he bears ultimate responsibility for Mr. Khashoggi’s death.