Replication stress, a hallmark of cancerous and pre-cancerous lesions, is linked to structuralchromosomal aberrations. Recent studies demonstrated that it could also lead to numericalchromosomal instability (CIN). The mechanism, however, remains elusive. Here, we showthat inducing replication stress in non-cancerous cells stabilizes spindle microtubules andfavours premature centriole disengagement, causing transient multipolar spindles that lead tolagging chromosomes and micronuclei. Premature centriole disengagement depends on theG2 activity of the Cdk, Plk1 and ATR kinases, implying a DNA-damage induced deregulationof the centrosome cycle. Premature centriole disengagement also occurs spontaneously insome CIN+cancer cell lines and can be suppressed by attenuating replication stress. Finally,we show that replication stress potentiates the effect of the chemotherapeutic agent taxol, byincreasing the incidence of multipolar cell divisions. We postulate that replication stress incancer cells induces numerical CIN via transient multipolar spindles caused by prematurecentriole disengagement.