The Justice Department's antitrust division unconditionally approved Google's $12.5 billion purchase of Motorola Mobility on Monday.
The move came just hours after the European Union's antitrust regulator also approved the deal.
The acquisition will give Google a foothold in the mobile phone industry and could boost its Android mobile operating system.
Google also gains access to Motorola’s trove of patents, which it can use to fend off lawsuits from Apple and other competitors.
Motorola split into two companies, Motorola Mobility and Motorola Solutions, early last year. Motorola Solutions, which provides data and communications tools to businesses and government agencies, will remain an independent company.
The Justice Department also announced that it had approved two patent deals. Apple, Microsoft and Research in Motion formed a partnership to buy 6,000 patents from Nortel Networks when the telecommunication company went bankrupt last year. Apple also bought patents from software firm Novell.
"After a thorough review of the proposed transactions, the Antitrust Division has determined that each acquisition is unlikely to substantially lessen competition and has closed these three investigations," the agency wrote in a statement. "In all of the transactions, the division conducted an in-depth analysis into the potential ability and incentives of the acquiring firms to use the patents they proposed acquiring to foreclose competitors."
The division noted that commitments from Apple and Microsoft to license essential patents to other companies on "fair, reasonable and non-discriminatory terms" helped to ease antitrust concerns.
"Google’s commitments were more ambiguous and do not provide the same direct confirmation of its [standard essential patents] licensing policies," the agency wrote.
The Justice Department said it would "not hesitate to take appropriate enforcement action stop any anticompetitive use" of the patents.