It depends on what you mean by "at war".
If you mean a state of war formally declared by Congress, no. The last war the US formally declared was against Bulgaria, Hungary, and Romania June 5th, 1942 as part of World War II. World War II was not a single declaration of war, but separate ones against Japan (Dec 8, 1941), Germany, (Dec 11, 1941 in response to their declaration of war against the US), Italy (same day, same reason) and finally Bulgaria, Hungary, and Romania June 5th, 1942.
The last formally declared war ended with the Treaty of San Francisco making peace with Japan coming into effect April 28th, 1952.
But it's not so simple. When a war formally "ends" also depends on how you define ending. For example, Bulgaria signed an Armistice with the Allies Oct 28, 1944 ending their fighting, but the peace treaties were signed in Paris in 1947. Similarly, Japan broadcast their acceptance of the Potsdam Declaration August 15th, 1945 ending the fighting, they formally Japan surrendered unconditionally in Sept 2nd, 1945, but a peace treaty wasn't signed until Sept 8th, 1951, and it didn't come into force until April 28th, 1952. One can argue that the war against Germany didn't formally end until the Treaty on the Final Settlement with Respect to Germany Sept 12, 1990.
If you mean an extended military engagement authorized by Congress, then the US is involved in quite a few. These are all authorized under the Authorization for Use of Military Force Against Terrorists (AUMF) of Sept 14, 2001 and broadly called "The War On Terror". What is being "at war" and what is "supporting" somebody else at war rapidly gets fuzzy. Similarly, the line between "counter-terrorism", "counter-insurgency", "counter-trafficking", and straight up getting involved in a civil war is also blurry.
If you mean military engagements authorized by United Nations Security Council Resolutions and funded by Congress, none that I'm aware of are ongoing. The last completed UN operation the US was officially involved in was intervention in the Libyan Civil War of 2011 authorized by UN Security Council Resolution 1973. It ended October 31, 2011 with the destruction of Gaddafi's regime.