The present invention provides a water-based ink that is excellent in adhesion to a resin printing medium formed of a resin such as polyethylene terephthalate, polyvinyl chloride, polyethylene, polypropylene, nylon, etc., gloss and anti-blocking properties upon storage of the printing medium after printing. The present invention relates to [1] a water-based ink including a colorant, polyester resin particles and modified polyolefin resin particles, in which the polyester resin particles are constituted of a resin containing an amorphous polyester having a glass transition temperature of not lower than 40° C. and not higher than 100° C., and a mass ratio of the polyester resin particles to the modified polyolefin resin particles (polyester resin particles/modified polyolefin resin particles) is not less than 10/90 and not more than 90/10, and [2] an ink-jet printing method including the steps of allowing the water-based ink according to the above [1] to adhere onto a resin printing medium by an ink-jet printing method; and then heating the resin printing medium onto which the water-based ink is adhered, to a temperature of not lower than 40° C. and not higher than 100° C.