A sheet binding device includes a first binding unit that performs a first binding process of binding a sheet bundle having a first thickness or less without using a staple, and a second binding unit that performs a second binding process of binding the sheet bundle having a second thickness or less exceeding the first thickness using a staple, the first binding process by the first binding device is preferentially executed when the thickness of the sheet bundle is the first thickness or less, and the second binding process by the second binding unit is executed when the thickness of the sheet bundle exceeds the first thickness and is equal to or less than the second thickness.