Exciton delocalization relates to many important photophysical processes such as excitation energy transfer, charge separation, and singlet fission. Here, we analyze the exciton delocalization through the photophysical measurements of the molecular crystal 2,2'-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(4-methylphenol) (m-MTTM), which is the segregated HJ-aggregate confirmed by the calculation of exciton coupling along each direction in the crystal structure. Linearly polarized steady-state absorption spectroscopy verifies that the red-shifted optical transition majorly arises from the aggregates unparalleled to the a-axis. In addition, the temperature-dependent emission spectra show the increase of 0-0 versus 0-1 vibronic emission ratio as the temperature decreases with the coherence number equaling 2.2-1.0 at 140-200 K, which is the characteristic behavior of J-aggregates. To elaborate these observations, we carry out the simulation with the Holstein-type Hamiltonian considering short-range charge-transfer-mediated couplings (perturbative regime) under the two-particle approximation, showing that the 3 × 3 laminar-like aggregates in the ac-plane and the 3 × 3 × 2 three-dimensional aggregates fit well with the emission spectrum at 140 K. In the 3 × 3 aggregates, the coherence function in the ac-plane shows the in-phase correlation along (1,0,-1), elucidating how J-aggregates form in segregated HJ-aggregates with dominant positive coupling. Under the strong intralayer out-of-phase correlation, the 3 × 3 × 2 aggregates demonstrate that the vibronic coupling has a great impact on the interlayer correlation. Furthermore, the coherence function along (0,1/2,-1/2) and (-1,1/2,-1/2) exhibits the thermal-activated phase flipping. These discoveries pave the ways for further manipulations of exciton delocalization in three-dimensional molecular solids.