Apparatuses, methods, and systems are disclosed for handling transport block (“TB”) size mismatch during RACH procedure. One apparatus includes a transceiver that receives a random access response message from a Radio Access Network (“RAN”) entity, wherein the random access response message grants an uplink resource allocation to the apparatus. The apparatus includes a processor that determines a size mismatch between a stored TB and the uplink resource allocation, where the stored TB comprises at least one medium access control (“MAC”) protocol data subunit (“subPDU”). In response to determining that there is a size mismatch, the processor generates a new TB that matches the size of the uplink resource allocation, wherein generating the new TB comprises appending at least one MAC subPDU.