U.S. and global economic growth would be boosted by increases in spending and tax cuts promised by President-elect Donald Trump, but those gains would be lost if he pressed ahead with threatened tariff increases that triggered retaliation, the Organization for Economic Cooperation and Development said Monday.

During his campaign, Mr. Trumppledged to boost infrastructure spending by as much as $1 trillion, although the details of how that would be financed are sketchy. He has also promised to cut corporate and personal income taxes.

In its twice-yearly report on global economic prospects, the Paris-based research body said that while the exact form it would take is uncertain, it does expect Mr. Trump to offer some fiscal stimulus from the early months of his presidency, and that its likely scale that would boost U.S. economic growth to 2.3% from 1.9% in 2017, and to 3% from 2.2% in 2018. There would also be benefits for other parts of the world as U.S. demand for imports rises, with global economic growth raised to 3.3% from 3.2% in 2017, and to 3.6% from 3.3% in 2018. The OECD is the first international economic policy agency to publish an estimate of the likely impact of Mr. Trump’s proposals.

“There’s now some prospect of the world exiting from this low-growth trap,” said Ángel Gurria, the OECD’s secretary-general. “Even though we still show the signs of those very heavy legacies of the crisis, we may be at a moment where we could see a turn for the better.”

The OECD’s forecasts are based on an increase in U.S. government spending in 2017 and 2018 of 0.25% of gross domestic product, a cut in income taxes that reduces government revenue by 0.5% of GDP in each year, and a cut in the corporate tax rate that reduces revenue by 0.75% of GDP in 2018. It calculates that thanks to the boost to growth, such a package would leave the government’s debts slightly smaller as a share of economic output.