The 29-year-old scored the USA's first, fifth, eighth, 10th and 12th goals, becoming just the second player in the history of the Women's World Cup to net five goals in a single game.
The record-setting night ended at 13-0.
If Tuesday is any indication, the United States appears to be well on its way to a second straight World Cup title and the team could very well set more tournament records along the way.
They would go on to score nine more, taking their tally of goals in the competition to 125, more than an eighth of the total scored by all teams all time. Seven different players (Alex Morgan, Rose Lavelle, Lindsey Horan, Sam Mewis, Megan Rapinoe, Mallory Pugh and Carly Lloyd) scored a goal and Morgan, the team's star striker, found the back of the net five times.
12-0, 87 minutes: Morgan's brilliant flick and powerful finish delivered the best goal of the night.
The defending champions have lost just one worldwide game since July 2017 and have won their past seven in a row, scoring 36 goals in the process, and have not conceded in any of their past five matches. As in 9-0 in favor of the United States.
Elsewhere, Sweden overcame hard conditions to beat a stubborn Chile side 2-0 in their opening game in Group F in Rennes.
The USA will face Chile in Paris at 17:00 BST on 16 June, three hours after Thailand meet Sweden.
The reigning champions had to wait five days to start their campaign but produced an ominous performance that doubled as the biggest win in tournament history, surpassing Germany's 11-0 trouncing of Argentina in 2007.
The top two teams in each of the six groups will qualify automatically for the last 16, along with the four best third-placed sides.