Portable thermal printer comprising: - a containment shell inside which the following is defined: a compartment designed to accommodate at least one roll of paper, an opening designed to allow said paper compartment to be accessed, a paper output opening or slot; - a cover designed to close said access opening, directly or indirectly hinged to said shell and suited to rotate around an axis from a closed position to an open position; - a printing mechanism housed in said containment shell and comprising at least one paper pressing roller positioned in proximity to said paper output opening or slot and held in that position through reversible constraint means; - a mechanism designed to release said paper pressing roller; wherein said reversible constraint means of the paper pressing roller are configured for the engagement of the rotation pin of the roller itself, and wherein the insertion/extraction of said pin in/from said constraint means is performed by translating said roller according to a direction, or exit direction, lying on a plane, or exit plane, and wherein said release mechanism is configured in such a way as to exert a force on said roller according to a direction which is parallel to said exit direction, and wherein said release mechanism comprises at least one arm, wherein one end of said arm is constrained to said paper pressing roller while a release button, accessible from the outside of the shell and positioned in proximity to said rotation axis of said cover, is connected to the opposite end of said arm, leaving the external area of said cover free; characterized in that said at least one arm is constrained in such a way that it slides with respect to said cover, when said cover is closed and when a user acts on said release button, between a first backward locking position and a second forward release position, along said exit direction, and in that the sliding movement of said arm pushes said paper pressing roller in said exit direction, extracting it from said constraint means. - a containment shell inside which the following is defined: a compartment designed to accommodate at least one roll of paper, an opening designed to allow said paper compartment to be accessed, a paper output opening or slot; - a cover designed to close said access opening, directly or indirectly hinged to said shell and suited to rotate around an axis from a closed position to an open position; - a printing mechanism housed in said containment shell and comprising at least one paper pressing roller positioned in proximity to said paper output opening or slot and held in that position through reversible constraint means; - a mechanism designed to release said paper pressing roller; wherein said reversible constraint means of the paper pressing roller are configured for the engagement of the rotation pin of the roller itself, and wherein the insertion/extraction of said pin in/from said constraint means is performed by translating said roller according to a direction, or exit direction, lying on a plane, or exit plane, and wherein said release mechanism is configured in such a way as to exert a force on said roller according to a direction which is parallel to said exit direction, and wherein said release mechanism comprises at least one arm, wherein one end of said arm is constrained to said paper pressing roller while a release button, accessible from the outside of the shell and positioned in proximity to said rotation axis of said cover, is connected to the opposite end of said arm, leaving the external area of said cover free; characterized in that said at least one arm is constrained in such a way that it slides with respect to said cover, when said cover is closed and when a user acts on said release button, between a first backward locking position and a second forward release position, along said exit direction, and in that the sliding movement of said arm pushes said paper pressing roller in said exit direction, extracting it from said constraint means.