Magnetic domains play a fundamental role in magnetization processes. However, unlike in ferromagnets (FMs), the formation of domains in antiferromagnets (AFMs) is poorly understood because they are not favored by magnetostatics and are difficult to detect experimentally. In this paper, we create a synthetic planar AFM with tunable lateral coupling between neighboring FM regions to establish the role played by magnetic disorder in the formation of AFM domains. By directly imaging the synthetic AFM in real space, we observe that the AFM lattice spontaneously breaks up into domains following ac demagnetization. These AFM domains nucleate and pin at locally disordered sites that define their size and shape, which is explained with the help of a Gaussian random field Ising model. Furthermore, we can manipulate the AFM domain morphology by varying the interaction strength, which can be tuned with the geometrical parameters of the synthetic AFM.