Ultralight dark matter (ULDM) is proposed as a theoretical candidate of dark matter particles with masses of approximately ${10}^{-22}\mathrm{eV}$. The interactions between ULDM particles and standard model particles would cause variations in pulse arrival times of millisecond pulsars, which means that the pulsar timing array (PTA) can be used to indirectly detect ULDM. In this Letter, we use the gamma-ray PTA composed of 29 millisecond pulsars observed by the Fermi Large Area Telescope (Fermi-LAT) to test four ULDM effects, including gravitational effects for generalized ULDM with different spin-$0/1$, the fifth-force coupling effect of dark photons, and the modified gravitational effect of the spin-2 ULDM. The gamma-ray pulsar timing is not affected by the ionized interstellar medium and suffers relatively simple noises, unlike that of the radio band. Our work is the first time that the gamma-ray PTA has been used to search for the ULDM. No significant signals of ULDM are found based on the Fermi-LAT PTA for all four kinds of ULDM models. Constraints on ULDM parameters are set with the 95% confidence level, which provides a complementary check of the nondetection of ULDM for radio PTAs and direct detection experiments.

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• Cosmic rays & astroparticles
• Gamma ray astronomy
• Gravitational waves
• Particle dark matter
• Neutron stars & pulsars
• Dark matter detectors
• Electromagnetic radiation astronomy
• Telescopes