Self-assembled quantum dots (SAQDs) grown under biaxial tension could enable novel devices by taking advantage of the strong band gap reduction induced by tensile strain. Tensile SAQDs with low optical transition energies could find application in the technologically important area of mid-infrared optoelectronics. In the case of Ge, biaxial tension can even cause a highly desirable crossover from an indirect- to a direct-gap band structure. However, the inability to grow tensile SAQDs without dislocations has impeded progress in these directions. In this article, we demonstrate a method to grow dislocation-free, tensile SAQDs by employing the unique strain relief mechanisms of (110)-oriented surfaces. As a model system, we show that tensile GaAs SAQDs form spontaneously, controllably, and without dislocations on InAlAs(110) surfaces. The tensile strain reduces the band gap in GaAs SAQDs by ~40%, leading to robust type-I quantum confinement and photoluminescence at energies lower than that of bulk GaAs. This method can be extended to other zinc blende and diamond cubic materials to form novel optoelectronic devices based on tensile SAQDs.