GTP) is a purine nucleotide. It can act as a substrate for the synthesis of RNA during the transcription process. Its structure is similar to that of the guanine nucleobase, the only difference being that nucleotides like GTP have a ribose sugar and three phosphates, with the nucleobase attached to the 1' and the triphosphate moiety attached to the 5' carbons of the ribose.

It also has the role of a source of energy or an activator of substrates in metabolic reactions, like that of ATP, but more specific. It is used as a source of energy for protein synthesis.GTP is essential to signal transduction, particularly with G-proteins, in second-messenger mechanisms where it is converted to GDP (guanosine diphosphate) through the action of GTPases.