To mix viscous fermentation broths more efficiently, a new close-clearance impeller, the rotational reciprocating plate impeller (RRPI), was designed to partly mimic the axial reciprocating plate impeller (ARPI). Akin to the axial movement of a reciprocating plate impeller, this new RRPI rotates back and forth driven by a three-arm linkage system. Its power consumption and overall mass transfer coefficient (K(L)a) were determined in a 22 L laboratory scale bioreactor using model fluids of different viscosities. Results obtained with this new impeller were compared to those of a triple Rushton impeller (TRI) and an ARPI. In water, results were the same as those of the TRI. However, the ARPI gave significantly lower K(L)a than the other two impellers for the same power input per unit volume. In highly viscous non-Newtonian carboxymethyl cellulose (CMC) solutions, the new impeller showed similar performance as the ARPI, whereas the TRI was not able to provide acceptable mixing due to the presence of stagnant zones. Therefore, this new impeller proves to be a suitable alternative to existing impellers for use in fermentations having rheologically evolving broth.