An ab initio study of the complexes formed by hypohalous acids (HOX, X = F, Cl and Br) with formaldehyde has been carried out at the MP2/aug-cc-pVTZ computational level. Two minima complexes are found, one with an H...O contact and the other one with an X...O contact. The former is more stable than the latter, and the strength difference between them decreases as the size of the X atom increases. The associated HO and XO bonds undergo a bond lengthening and red shift, whereas a blue shift was observed in the bond of the hypohalous acid not involved in the interaction. The interaction strength and properties in both complexes are analyzed with atoms in molecules (AIM) and natural bond orbital (NBO) theories. The energy decomposition analyses indicate that the contribution from the electrostatic interaction energy is larger in the hydrogen-bonded complexes than that in the halogen-bonded complexes.