A heat exchanger for an EGR system adapted for the heat exchange between a first fluid, the exhaust gas of an internal combustion engine, and a second fluid, a liquid coolant, comprising: - a shell with an inlet (1.1) and an outlet (1.2) for the second fluid; - a heat exchange tube bundle housed inside the shell formed by stacking flat tubes (2.1) having a rectangular section, arranged parallel to one another, extending according to a longitudinal direction (X-X') between an inlet of the first fluid and an outlet of the first fluid; wherein the space between the exchange tube bundle and the shell is configured for the passage of the second fluid; and wherein the flat tubes (2.1) of the tube bundle comprise an expansion (2.1.1), in the direction of the stack of the tube bundle, at the ends thereof to establish a passage space between tubes (2.1) for the second fluid; and wherein at least one of the tubes (2.1) of the bundle tubes: is configured by attaching two flat plates with bent sides (2.1.5), such that an inner face of the bent side (2.1.5) of a plate is attached to the outer face of the bent side (2.1.5) of the other plate; wherein both plates have groups of first protrusions (2.1.2) distributed along the longitudinal direction (X-X'), characterized in that at least one plate has one or more second protrusions (2.1.3) deeper than the first protrusions (2.1.2) that reach the opposite plate, both plates being either in contact with one another or being attached by means of the at least one second protrusions, forming longitudinal channels (2.1.6) inside the flat tube (2.1), and wherein, given the transverse direction (Y-Y') as the perpendicular direction with respect to the longitudinal direction (X-X') contained in the main plane of the flat tube (2.1), the second protrusion or protrusions (2.1.3) have either projections (2.1.3.1) in the transverse direction (Y-Y') or deviations (2.1.3.2) in the transverse direction (Y-Y'), or both, for disturbing the flow of the first fluid in the transverse direction (Y-Y') from the walls of the channel (2.1.6) formed by said second protrusions (2.1.3). - a shell with an inlet (1.1) and an outlet (1.2) for the second fluid; - a heat exchange tube bundle housed inside the shell formed by stacking flat tubes (2.1) having a rectangular section, arranged parallel to one another, extending according to a longitudinal direction (X-X') between an inlet of the first fluid and an outlet of the first fluid; wherein the space between the exchange tube bundle and the shell is configured for the passage of the second fluid; and is configured by attaching two flat plates with bent sides (2.1.5), such that an inner face of the bent side (2.1.5) of a plate is attached to the outer face of the bent side (2.1.5) of the other plate; wherein both plates have groups of first protrusions (2.1.2) distributed along the longitudinal direction (X-X'), characterized in that at least one plate has one or more second protrusions (2.1.3) deeper than the first protrusions (2.1.2) that reach the opposite plate, both plates being either in contact with one another or being attached by means of the at least one second protrusions, forming longitudinal channels (2.1.6) inside the flat tube (2.1), and wherein, given the transverse direction (Y-Y') as the perpendicular direction with respect to the longitudinal direction (X-X') contained in the main plane of the flat tube (2.1), the second protrusion or protrusions (2.1.3) have either projections (2.1.3.1) in the transverse direction (Y-Y') or deviations (2.1.3.2) in the transverse direction (Y-Y'), or both, for disturbing the flow of the first fluid in the transverse direction (Y-Y') from the walls of the channel (2.1.6) formed by said second protrusions (2.1.3).