We explore the interannual variability of summertime extratropical transient activity and assess its influence on rainfall over southeastern South America focusing on Uruguay. Transient wave activity is characterized calculating the group velocity using daily mean meridional velocity at 300 hPa. It is found that transient activity shows strong interannual variability consisting in patterns that represent a strengthening of wave activity in the subtropical Pacific, and meridional shifts in the position of the Pacific and Atlantic storm tracks. Part of the variability is associated with El Niño, which strongly influences northern Uruguay. Rainfall in southern Uruguay is correlated with latitudinal shifts in transient wave activity such that a strengthening in transient energy between 35∘ and 45∘S extending from the southeastern Pacific into the South American continent favours positive anomalies. For El Niño to induce increased rainfall over southern Uruguay it has to force two stationary waves: one that emanates from central Pacific describing an arch of low curvature into high latitudes and then towards the south Atlantic and another shorter wave that emanates from the eastern Pacific turning north-eastward at the latitude of the subtropical jet. When the latter is strong, surface northerlies reach southern Uruguay destabilizing the atmosphere due to enhanced transport of moisture and warm air. Coincident changes in transient wave activity south of 35∘S increases the associated frontal activity and thus can induce positive rainfall anomalies by tapping the increased available moisture. In this case, the sea surface temperature (SST) anomalies are maximum in the central equatorial Pacific. On the other hand, a warmer eastern Pacific generates a stationary wave that shifts the subtropical jet northward increasing rainfall over south Brazil and decreasing it to the south of Uruguay. Thus, the summertime influence of El Niño over southern Uruguay is strongly dependent on the pattern of equatorial Pacific SST anomalies.