In genetics, GC-content (guanine-cytosine content) is a characteristic of the genome of any given organism or any other piece of DNA. It is the proportion of GC-base pairs in the DNA. G stands for guanine and C stands for cytosine. GC-pairs in the DNA are connected with three hydrogen bonds instead of two in the AT-pairs (adenine and thymine). This makes the GC-pair stronger and more resistant to denaturation by high temperatures.

The GC-content is sometimes used to classify organisms in taxonomy, for example, the Actinomycetales bacteria are characterised as "high GC-content bacteria". In Streptomyces coelicolor it is 72%. The GC-content of Yeast (Saccharomyces cerevisiae), for example, is 38%, that of another common model organism Thale Cress (Arabidopsis thaliana) is 36%.

In PCR the GC-content of primers is used to determine annealing temperature.

The GC-content can be measured by several means but one of the simplest methods is to measure what is called the melting temperature of the DNA double helix with a spectrophotometer. The absorbance of DNA at a wavelength of 260 nm increases fairly sharply when the double-stranded DNA separates into two single strands when sufficiently heated.