The two blocs won first and second place in the war-scarred country's May 12 parliamentary election.
The move by Sadr, who is staunchly opposed to Iranian involvement in the country, was unexpected by much of the political class as he had suggested unwillingness to work with Ameri and his bloc of pro-Iranian former fighters.
But at a joint press conference with Ameri in Al Najaf, Sadr hailed the formation of "a true alliance to accelerate the formation of a national government away from any dogmatism".
Confusion has gripped Iraq since Sadr's electoral alliance with communists won the vote. Last week Iraq's parliament ordered a manual vote recount and sacked the commission which oversaw the polls amid mounting claims of electoral fraud.