The "reciprocal" rates from the Trump administration, which can reach up to 50%, will be levied on some important manufacturing centres in Asia.
Numerous countries he vowed to respond, and China has already retaliated with new tariffs of its own on goods imported from the US. Trump on Monday threatened to put an additional 50% tariff on goods from the country, which would bring the total taxes he intends to charge to more than 100%.
Trump had launched an "economic war against the whole world at once" that risked shattering investor confidence in the US, Mr Ackman commented.
Mr Ackman said the American leader now had "an opportunity to call a 90-day time out, negotiate and resolve unfair asymmetric tariff deals, and induce trillions of dollars of new investment in our country".
His post on Sunday indicated that he felt the ball was back in Trump's court - after an earlier message on X which urged leaders of other countries to "pick up the phone" to make a deal with Trump.
As stock markets around the world continued their slump on Monday, the head of banking giant JPMorgan Chase offered his own take, warning of "many uncertainties" around the new tariffs policy.
In a letter to shareholders, Mr Dimon said the tariffs will "likely increase inflation and are causing many to consider a greater probability of a recession".
"The quicker this issue is resolved, the better because some of the negative effects increase cumulatively over time and would be hard to reverse," he wrote.
Trump's officials he downplayed the recession risk. The baseline 10% tariff is already in effect, with the higher rates faced by some countries due to come into effect on Wednesday.
Speaking aboard the presidential plane on a flight back to Washington DC on Sunday, Trump himself said European and Asian countries were "dying to make a deal".