This is a list of international treaties the United States has either not signed, not ratified, or signed/ratified but later withdrawn its signature/ratification from.
"[The President] shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two-thirds of the Senators present concur..." The Treaty Clause in Article Two of the United States Constitution dictates that the President of the United States negotiates treaties with other countries or political entities, and signs them.
Signed treaties enter into force only if ratified by at least two-thirds (67 members) of the United States Senate.
(Technically, the Senate itself does not ratify treaties, it only approves or rejects resolutions of ratification submitted by the Committee on Foreign Relations; if approved, the United States exchanges the instruments of ratification with the foreign power(s)).
[4] According to a 2014 analysis by The New Republic, the ratification of a significant number of treaties signed after 1990 has been blocked by senators of the Republican Party for various ideological reasons.