Context : The stellar mass function is a probe for a potential dependence of star formation on the environment .
Only a few young clusters are known to reside within the Central Molecular Zone and can serve as testbeds for star formation under the extreme conditions in this region .

Aims : We determine the present-day mass function of the Quintuplet cluster , a young massive cluster in the vicinity of the Galactic centre .

Methods : We use two epochs of high resolution near infrared imaging data obtained with NAOS/CONICA at the ESO VLT to measure the individual proper motions of stars in the Quintuplet cluster in the cluster reference frame .
An unbiased sample of cluster members within a radius of 0.5 \mathrm { pc } from the cluster centre was established based on their common motion with respect to the field and a subsequent colour-cut .
Initial stellar masses were inferred from four isochrones covering ages from 3 to 5 Myr and two sets of stellar evolution models .
For each isochrone the present-day mass function of stars was determined for the full sample of main sequence cluster members using an equal number binning scheme .

Results : We find the slope of the present-day mass function in the central part of the Quintuplet cluster to be \alpha = -1.66 \pm 0.14 for an approximate mass range from 5 to 40 \mathrm { M _ { \sun } } , which is significantly flatter than the Salpeter slope of \alpha = -2.35 .
The flattening of the present-day mass function may be caused by rapid dynamical evolution of the cluster in the strong Galactic centre tidal field .
The derived mass function slope is compared to the values found in other young massive clusters in the Galaxy .

Conclusions :