A diamond single crystal according to the present invention is a diamond single crystal synthesized using a chemical vapor synthesis method and has an absorption coefficient of 25 cm -1 or more and 80 cm -1 or less for light having a wavelength of 350 nm. A method for producing a diamond single crystal according to the present invention includes implanting an ion other than carbon into a main surface of a diamond single crystal seed substrate and thereby decreasing the transmittance of light having a wavelength of 800 nm, the main surface having an off-angle of 7 degrees or less with respect to a {100} plane, and homoepitaxially growing a diamond single crystal on the ion-implanted main surface of the seed substrate using a chemical vapor synthesis method under synthesis conditions where the ratio N C /N H of the number of carbon-containing molecules N C to the number of hydrogen molecules N H in a gas phase is 10% or more and 40% or less, the ratio N N /N C of the number of nitrogen molecules N N to the number of carbon-containing molecules N C in the gas phase is 0.1% or more and 10% or less, and the seed substrate temperature T is 850°C or more and less than 1000°C.