In anthropology and demography, the human sex ratio is the sex ratio for Homo sapiens (i.e., the ratio of males to females in a population). Like most sexual species, the sex ratio is approximately 1:1. In humans the secondary sex ratio (i.e., at birth) is commonly assumed to be 105 boys to 100 girls, an assumption that is a subject of debate in the scientific community. The sex ratio for the entire world population is 101 males to 100 females.

Gender imbalance may arise as a consequence of various factors ranging from natural factors and war casualties to intentional gender control and deliberate gendercide.

More data are available for humans than for any other species, and the human sex ratio is more studied than that of any other species, but interpreting these statistics can be difficult.

Human sex ratios, either at birth or in the population as a whole, might be quoted in any of four ways: the ratio of males to females, the ratio of females to males, the proportion of males, or the proportion of females. If there are 108,000 males and 100,000 females, the ratio would, respectively, be quoted as 1.080, 0.926, 0.519 or 0.481. Sex ratio in scientific literature is often expressed as the proportion of males. In contrast, sex ratio quoted in this article is the ratio of males to females, unless specified otherwise.