During the last decades, the spectral wave period Tm-1,0 has become accepted as a characteristic wave period when describing the hydraulic attack on coastal structures, especially over shallow foreshores. In this study, we derive an empirical prediction formula for Tm-1,0 on shallow to extremely shallow foreshores with a mild slope. The formula was determined based on flume tests and numerical calculations, mainly for straight linear foreshore slopes. It is shown that the wave period increases drastically when the water depth decreases; up to eight times the offshore value. The bed slope angle influences the wave period slightly. For short-crested wave fields, the strong increase of Tm-1,0 starts closer to shore (at smaller water depths) than for long-crested wave fields.