Nucleotides are organic molecules that serve as the monomer units for forming the nucleic acid polymers deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules in all life-forms on Earth. Nucleotides are the building blocks of nucleic acids; they are composed of three subunit molecules: a nitrogenous base, a five-carbon sugar (ribose or deoxyribose), and at least one phosphate group. They are also known as phosphate nucleotides.

A nucleoside is a nitrogenous base and a 5-carbon sugar. Thus a nucleoside plus a phosphate group yields a nucleotide.

Nucleotides also play a central role in life-form metabolism at the fundamental, cellular level. They carry packets of chemical energy—in the form of the nucleoside triphosphates ATP, GTP, CTP and UTP—throughout the cell to the many cellular functions that demand energy, which include synthesizing amino acids, proteins and cell membranes and parts; moving the cell and moving cell parts, both internally and intercellularly; dividing the cell, etc. In addition, nucleotides participate in cell signaling (cGMP and cAMP), and are incorporated into important cofactors of enzymatic reactions (e.g. coenzyme A, FAD, FMN, NAD, and NADP+).

In experimental biochemistry, nucleotides can be radiolabeled with radionuclides to yield radionucleotides.

A nucleotide is composed of three distinctive chemical sub-units: a five-carbon sugar molecule, a nitrogenous base—which two together are called a nucleoside—and one phosphate group. With all three joined, a nucleotide is also termed a "nucleoside monophosphate". The chemistry sources ACS Style Guide and IUPAC Gold Book prescribe that a nucleotide should contain only one phosphate group, but common usage in molecular biology textbooks often extends the definition to include molecules with two, or with three, phosphates. Thus, the terms "nucleoside diphosphate" or "nucleoside triphosphate" may also indicate nucleotides.