In this work, the effect of Trametes pubescens laccase (TpL) used in combination with a low-molecular-weight ultra-filtered lignin (UFL) to improve mechanical properties of kraft liner pulp and chemi-thermo-mechanical pulp was studied. UFL was isolated by ultra-filtration from the kraft cooking black liquor obtained from softwood pulping. This by-product from the pulp industry contains an oligomeric lignin with almost twice the amount of free phenolic moieties than residual kraft pulp lignin. The reactivity of TpL on UFL and kraft pulp was studied by nuclear magnetic resonance spectroscopy and size exclusion chromatography. Laccase was shown to polymerise UFL and residual kraft pulp lignin in the fibres, seen by the increase in their average molecular weight and in the case of UFL as a decrease in the amount of phenolic hydroxyls. The laccase initiated cross-linking of lignin, mediated by UFL, which gives rise to more than a twofold increase in wet strength of kraft liner pulp handsheets without loosing other critical mechanical properties. Hence, this could be an interesting path to decrease mechano-sorptive creep that has been reported to lessen in extent as wet strength is given to papers. The laccase/2,2'azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) mediator system showed a greater increase in wet tensile strength of the resulting pulp sheets than the laccase/UFL system. However, other mechanical properties such as dry tensile strength, compression strength and Scott Bond internal strength were negatively affected by the laccase/ABTS system.