Surface integrity remains a significant concern in the additive manufacturing (AM) of titanium alloys, particularly due to implications for fatigue properties in critical applications, such as aerospace and biomedical. To address this concern, the current research investigates the effectiveness of ultrasonic pulsed waterjet (UPWJ) peening as a novel mechanical treatment for enhancing the surface characteristics of laser powder bed fusion (L-PBF) Ti–6Al–4V specimens. For UPWJ peening, a jet pressure of 69 MPa and a traverse speed of 800 mm/s were utilized, at a pulse frequency of 40 kHz. In addition to L-PBF, wrought Ti–6Al–4V specimens were also subjected to peening under identical UWPJ conditions, in order to assist meaningful comparisons. A comprehensive assessment of surface properties, including microstructure, surface roughness, residual stress, and scratch hardness, was conducted. The results indicated a significant reduction in the arithmetic mean height (Ra) for L-PBF components post-UPWJ peening, yielding a more consistent surface topography. Moreover, UPWJ peening induced substantial compressive residual stresses (approximately −800 MPa) uniformly across both AM and wrought samples. In terms of mechanical response, this treatment led to a notable 20% enhancement in scratch hardness (HSp). In conclusion, UPWJ peening showed the capability to offer improved surface properties and great magnitudes of compressive residual stress, positioning it as a feasible post-processing technique for enhancing the quality of PBF-LB fabricated Ti–6Al–4V components.