We demonstrate the generation of high-energy picosecond pulses directly from a thin-disk laser oscillator by employing a self-imaging active multipass geometry. Stable single-pulse operation has been obtained with an average output power in excess of $50\mathrm{W}$ , excluding a cw background of 8%, at a repetition rate of $3.8\mathrm{MHz}$ . Self-starting passive mode locking was accomplished using a semiconductor saturable absorber mirror. The maximum pulse energy was $13.4\mu \mathrm{J}$ at a pulse duration of $1.36\mathrm{ps}$ with a time-bandwidth product of 0.34. Single-pass external frequency doubling with a conversion efficiency of 60% yielded $>28\mathrm{W}$ of average power at $515\mathrm{nm}$ .