The case, filed in New York, claims that mobile prices would rise if the third- and fourth-biggest wireless outfits in the country were allowed to become one, The Wall Street Journal reported.
As iTWire reported, FCC chairman Ajit Pai gave his blessing to the deal in May. This came following national security clearance after both Deutsche Telekom, the majority owner of T-Mobile, and Sprint's majority owner, Japanese firm SoftBank, agreed to exclude Chinese telecommunications giant Huawei as a supplier.
Both companies also pledged to build 5G infrastructure in rural areas and sell off Sprint's Boost Mobile pre-paid mobile service.
It is unusual for state officials to file legal challenges of this kind unless they are support by their federal counterpart.
T-Mobile and Sprint announced their proposed merger in April last year. Since then, the FCC has been provided with a huge amount of feedback from parties who have an interest in the transaction.
The US Department of Justice has yet to give its final word on the deal.
A previous effort to lessen the number of national US wireless operators down to three failed in 2011, when the Justice Department and the FCC both opposed AT&T's plan to buy T-Mobile, saying it would lessen competition.