Jacinda Ardern has re-entered the political fray with a rallying call for internationalism, rebuking the inward outlook of the United States under President Donald Trump.

The popular former New Zealand prime minister spoke at Yale College's Class Day on Monday (Australian time), the undergraduate arm of the prestigious Ivy League university.

Dame Jacinda, who has lived in the US as a Harvard-based fellow since late 2023, said she opted against 'the usual pep talk that perhaps you might expect' in an address witnessed by thousands.

'Suddenly didn't feel enough. Not when the world, over the course of a few short months, moved from tumultuous to an all-out dumpster fire,' she said.

'There are challenges to rules around trade, increases in migration flows and a decreasing regard for civil rights and human rights, including the right to be who you are,' she said.