Sperm performance is often tightly linked to male reproductive success. In many demersal gobiid fishes, the male attaches sperm embedded in a mucus produced by sperm duct glands, to the nest substrate before spawning takes place. Sperm are activated as the mucus and other embedded gland contents dissolve into the water. To test the importance of gland content on sperm function in Pomatoschistus minutus, a marine fish with external fertilization, we used a paired experimental design, with spermatozoa tested with and without sperm duct gland contents mixed into seawater. We measured sperm velocity, percentage motile sperm and sperm viability over time. Sperm were found to swim 7.3% faster when gland contents were mixed in the seawater. Percentage motile sperm was unaffected by the gland contents. Sperm viability in seawater exceeded 24 h, but was unaffected by the gland contents. An increase in sperm velocity of similar magnitude as found here has been shown by others to increase reproductive success. Since velocity-boosting properties of sperm duct gland contents have now been found in three distantly related goby species, these traits may be conserved across the Gobiidae family and may aid in reproduction across a range of species and environments.