The importation of slaves into the United States was prohibited by an Act of Congress, signed into law by then Pres. Jefferson, in Mar. 1807, and effective on 1 Jan. 1808, the earliest date allowable under the U.S. Constitution, Article I, Section 9.
Actually slave trade continued rather fiercly after the act on Congress outlawing it:
The Legacy of 1808: Post-Abolition Slave Trade
It is difficult to explain why it was moralist sentiment was not strong enough to carry the day. One possible explanation is that even though there was strong sentiment to abolish the trade in Congress, constituencies in the South were able to exert sufficient pressure to weaken the force of the law. The delegates to the Constitutional Convention could not have forseen the effect that Ely Whitney’s cotton gin would have on Southern agriculture. The decades following the abolition of the slave trade show that United States did not have enough will to even enforce the laws they had passed. Illegal slave trade continued overland through Texas and Florida, while ships continued to smuggle slaves in through South Carolina.27 Even though Congress passed a law in 1820 making participation in the slave trade an act of piracy and punishable by death, it was not strongly enforced.
In the 1820’s, the nature of the illegal slave trade changed somewhat. US ships were now primarily involved in the transport of slaves from Africa to other countries in North and South America like Cuba and Brazil. The British wanted cooperation from the Americans in the form of the mutual right of search and seizure. The Americans opposed this principle, not so much out of a desire to continue the slave trade, but out of a sense of national pride and an appeal to the freedom of the seas.28 The US’s refusal to enforce its own anti-slave trade laws, as well as cooperate with other nations allowed the slave trade to continue for decades to come.