InAs in-plane ultrahigh-density quantum dots (IP-UHD QDs) were grown on GaAsSb/GaAs(001) by molecular beam epitaxy and introduced into the active layer of a ridge-waveguide AlGaAs/GaAs laser. The IP-UHD QD density was 5 × 1011 cm−2. Despite having a short cavity length, no high-reflective coating on the cavity edge and a small number of stacked QD layers, stable laser operation up to 80 ℃ has been achieved. IP-UHD QD lasers without p-type doping exhibited a characteristic temperature of 77 K. IP-UHD QD lasers have the same low internal loss as conventional QD lasers. Improved uniformity in IP-UHD QDs promises the achievement of ultralow threshold current.