30 January 2017 Last updated at 17:58 GMT
The new rules, brought in on Friday, also include stopping Syrian refugees travelling to the US until further notice.
Mr Trump said the plans were about keeping America safe from terrorism, and it was not a ban against Muslims.
There have been big protests against the ban over the weekend.
Leah's got everything you need to know.
US President Donald Trump is continuing to back a ban on people from seven mainly Muslim countries entering America, despite protests against it.