Topological phases enable protected transport along the edges of materials, offering immunity against scattering from disorder and imperfections 1 . These phases were suggested and demonstrated not only for electronic systems, but also for electromagnetic waves 2-9 , cold atoms 10-12 , acoustics 13 , and even mechanics 14 . Their potential applications range from spintronics and quantum computing to highly efficient lasers. Traditionally, the underlying model of these systems is a spatial lattice in two or three dimensions. However, it recently became clear that many lattice systems can exist also in synthetic dimensions which are not spatial but extend over a different degree of freedom 15,16 . Thus far, topological insulators in synthetic dimensions were demonstrated only in cold atoms [17][18][19] , where synthetic dimensions have now become a useful tool for demonstrating a variety of lattice models that are not available in spatial lattices [20][21][22][23][24] . Subsequently, efforts have been directed towards realizing topological lattices with synthetic dimensions in photonics, where they are connected to physical phenomena in high-dimensions, interacting photons, and more [25][26][27][28][29] . Here we demonstrate experimentally the first photonic topological insulator in synthetic dimensions. The ability to study experimentally photonic systems in synthetic dimensions opens the door for a plethora of unexplored physical phenomena ranging from PT-symmetry 30,31 , exceptional points [31][32][33] and unidirectional invisibility 34 to Anderson localization in high dimensions and high-2 dimensional lattice solitons 16 , topological insulator lasers in synthetic dimensions 35,36 and more. Our study here paves the way to these exciting phenomena, which are extremely hard (if not impossible) to observe in other physical systems.In the field of topological insulators, one of the most striking phenomena is the appearance of topological edge-states 1 . Topological edge-states are eigenstates of the system that reside in a topological band gap, are robust to disorder, immune to backscattering from defects and are localized at the edge of the lattice. In light of these properties, transport via topological edge-states is important both for its fundamental aspects and for potential technological applications. The concepts underlying topological edge-states extend much beyond condensed matter. In fact, topological edge-states were demonstrated in a variety of physical systems, ranging from electromagnetic waves both in the microwave regime 3,37 and at optical frequencies 8,9 , to cold atoms 11,12 , acoustics 13 and even mechanical systems 14 . Despite of their different manifestations, all of these topological insulators systems rely on spatial lattices, and the wavepackets occupying the lattice, whether they are electrons, photons or phonons, are subjected to gauge fields that give rise to the topological phenomena.However, lattices may take other forms than a spatial arrangement of sites: they can be assigned to a l...