Wood flour or sawdust is often used as filler in conventional wood plastic composite (WPC) materials. However, there has been an increasing interest to the use of wood pulp fibres in reinforced plastic applications, because they can provide enhanced strength properties and better biodegradability characteristics for the composite. This research compares the effect of recycled fibres or side streams of paper as reinforcement in polylactic acid (PLA) or polypropylene (PP) composites. Fibre material from liquid packaging board, non-deinked old newspapers and fibre sludge from recycling processes are compared with virgin softwood kraft pulp fibres. Composites were produced by melt processing to a fibre content of 30 wt.% (or 10 wt.% fibre sludge), and the mechanical properties were investigated. Recycled fibres provided comparable, or even higher, plastic reinforcement than virgin softwood fibres. In polypropylene composites, the differences in mechanical properties between different fibre types were relatively small. Fibre sludge decreased the mechanical performance of composites but can be considered as cheap filler in cases when mechanical properties are not crucial. The possibility to use low-cost materials like recovered paper or deinking sludge in wood plastic composites is an interesting option for future sustainable applications.