We report on the first demonstration, to the best of our knowledge, of a Ti:sapphire thin-disk laser oscillator, and discuss the thermal behavior and the thermally induced lens for different sizes of the pump spot. In multimode cw operation a maximum output power of 7 W was achieved with an optical efficiency of 11%. The emission spectrum exhibits a spectral width of about 10 nm FWHM centered at a wavelength of 790 nm.