Increasing the wood density to a preferred range contributes to upgrading the value of wood as raw material. Lignin manipulation can also improve wood quality. This study attempted to accelerate secondary cell wall formation in transgenic poplar with an altered lignin structure. To achieve this,OsSWN1, a rice master switch for secondary cell wall formation in fiber cells, was overexpressed in poplar plants in which monolignol biosynthesis was suppressed by the expression of an RNA-interference construct targeted to the gene for cinnamyl alcohol dehydrogenase 1. The generated transgenic poplars successfully overexpressed the chimericOsSWN1construct and monolignol biosynthesis remained altered in these plants. Secondary cell wall thickness was increased in the transgenic plants and their wood density was higher compared to the background line. No difference in lignin content was observed, except in one transgenic line. High saccharification characteristics observed in the background line were preserved in the transgenic line with lowerOsSWN1overexpression, but higherOsSWN1overexpression had a slight negative impact on enzyme saccharification. Our data suggested that fine-tuning of accelerated deposition of the secondary cell wall combined with alteration of monolignol biosynthesis should improve the lignocellulose quality for conventional and future biorefinery uses.