Lopez Obrador said he was hopeful that President-elect Joe Biden would agree to work with Mexico and other countries on the issue, BBC reported.
About 7,000 migrants, mostly from Honduras, have entered Guatemala.
They hope to travel on to Mexico and eventually reach the US border.
Every year, tens of thousands of Central American migrants try to reach the US, often on foot, in groups known as "caravans".
They say they are fleeing persecution, violence and poverty in their home countries. Conditions have been made worse by the devastation wrought by two huge hurricanes that battered Central America last November.
In remarks on Monday, Lopez Obrador urged the US to reform its policies on immigration.
"I think the time has come for the commitment [to immigration reform] to be fulfilled, and that is what we hope," he said.
"In Joe Biden's campaign, he offered to finalize immigration reform and I hope that he is able to achieve this. That is what I hope."
He said his government would try to dissuade migrants from crossing into Mexico but added that the rights of all migrants must be respected.
In Guatemala on Monday, security forces broke up a caravan of about 4,000 mostly Honduran migrants who had been camped out near the village of Vado Hondo.
Witnesses said officers, beating their batons against their shields, tried to force the group back in the direction of the Honduran border, about 50km (31 miles) away.
The migrants scattered but several threw stones at police who responded by firing tear gas.
The caravan had been held back in the area since Saturday and was blocking a key road, causing a long tail-back of traffic. Clashes broke out on Sunday as some of the migrants tried to force their way past police lines.
Speaking to reporters, Guatemalan Foreign Minister Pedro Brolo urged the Honduran government to help ensure "an orderly and safe passage home for those in this caravan".
President Donald Trump has taken a hard line against illegal immigration, especially along the southern US border with Mexico. He has also put pressure on Mexico, Guatemala, Honduras and El Salvador to crack down on northbound migrants.
Biden has vowed to end the strict immigration policies of his predecessor but his administration, which will take office on Wednesday, has warned migrants not to make the journey because policies will not change overnight.
An administration official told NBC News that migrants trying to claim asylum in the US "need to understand they're not going to be able to come into the United States immediately."
The Biden administration will prioritize undocumented immigrants already living in the US, not those heading to the country now, the official said.