A modular inkjet printhead assembly including a plurality of printhead modules mounted on alternating sides of a central rail assembly. The rail assembly includes a beam and a rod attached to a side of the beam. The printhead modules include a jetting module having an array of nozzles, a first alignment tab having a first alignment datum and a second alignment datum, a second alignment tab having a third alignment datum and a fourth alignment datum, a rotational alignment feature including a fifth alignment datum, and a cross-track alignment feature including a sixth alignment datum. Portions of the alignment tabs of the jetting module are adapted to fit within corresponding notches in the beam. A jetting module clamping mechanism and a jetting module cross-track force mechanism apply forces to the jetting module that causes each alignment datum to engage with corresponding alignment features on the rail assembly.