The Trump administration’s decision to continue trade negotiations with allies averted new tariffs but threatens to slow corporate spending and drive up costs, developments that could inject new uncertainty into financial markets.

The questions about tariffs—and when they would take effect—already have been rattling Wall Street. Investors dumped stocks when they feared the chances of a trade war were escalating, only to buy back in when those worries subsided. Since the tariffs were announced in early March, the S&P 500 has...