Yes it is legal. The U.S. Treasury classified bitcoin as a convertible decentralized virtual currency in 2013.[28] The Commodity Futures Trading Commission, CFTC, categorized bitcoin as a commodity in September 2015. Per IRS, bitcoin is taxed as a property.

Bitcoin was cited in a U.S. Supreme Court opinion (on Wisconsin Central Ltd. v. United States) regarding the altering definition of cash on 21 June 2018.

If money offerings businesses, which includes cryptocurrency exchanges, money transmitters, and anonymizing services (known as “mixers” or “tumblers”) do a substantial amount of business in the U.S., they are required to

register with the U.S.FinCEN as a money services business
design and enforce an anti-money laundering (AML) program, and
keep appropriate documents and make reports to FinCEN, inclusive of Suspicious Activity Reports (SARs) and Currency Transaction Reports (CTRs).

Seventeen other nations have comparable AML requirements. As of 2018 U.S. FinCEN receives greater than 1,500 SARs per month involving cryptocurrencies.

In September 2016, a federal judge ruled that “Bitcoins are funds within the simple meaning of that term”.