Problem gambling rates in older adults have risen dramatically in recent years and require further investigation. Limited available research has suggested that social needs may motivate gambling and hence problem gambling in older adults. Un-partnered older adults may be at greater risk of problem gambling than those with a partner. The current study explored whether loneliness mediated the marital status–problem gambling relationship, and whether gender moderated the mediation model. It was hypothesised that the relationship between being un-partnered and higher levels of loneliness would be stronger for older men than older women. A community sample of Australian men (n = 92) and women (n = 91) gamblers aged from 60 to 90 years (M = 69.75, SD = 7.28) completed the UCLA Loneliness Scale and the Problem Gambling Severity Index. The results supported the moderated mediation model, with loneliness mediating the relationship between marital status and problem gambling for older men but not for older women. It appears that felt loneliness is an important predictor of problem gambling in older adults, and that meeting the social and emotional needs of un-partnered men is important.