Do I need a visa to enter Mainland China?

I am flying in to HK with a USA passport, then I am taking the ferry to Macau. Then from Macau I will be traveling to Mainland PRC from Macau by car for couple of days. If I do need an entry permit for Mainland PRC can I apply from Macau or HK?
From what I know you don't need entry permit for HK, Macau but I am not sure about Mainland PRC.
Thanks