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A highly underexpanded jet outflow into the background in transition and scattering regimes is studied computationally. The direct simulation Monte Carlo method and Navier–Stokes equations are used. The main parameters’ impact on the jet flow is analyzed. It is shown that a drastic flow structure transformation occurs in a relatively narrow Reynolds numbers’ range, 5 ≤ ReL ≤ 30, featuring the jet–surrounding gas interaction. At ReL = 5, a shock wave structure that is typical for the underexpanded jet degenerates completely. The existing empirical expressions application for the estimation of the characteristic dimensions of the shock wave structure in the transition regime leads to significant inaccuracy. For the considered parameters’ range, the approaches based on the direct simulation Monte Carlo method and Navier–Stokes (NS) equations’ solution lead to similar results in the nozzle region, where the flow regime is hydrodynamic. Nevertheless, the NS approach employment for the assessment of flow parameters within rarefied shock layers is debatable.