A substantial strengthening of the South American monsoon system (SAMS) during Heinrich Stadials (HS) points toward decreased cross-equatorial heat transport as the main driver of monsoonal hydroclimate variability at millennial time scales. In order to better constrain the exact timing and internal structure of HS1 over tropical South America, we assessed two precisely dated speleothem records from central-eastern and northeastern Brazil in combination with two marine records of terrestrial organic and inorganic matter input into the western equatorial Atlantic. During HS1, we recognize at least two events of widespread intensification of the SAMS across the entire region influenced by the South Atlantic Convergence Zone (SACZ) at 16.11-14.69kyr B.P. and 18.1-16.66kyr B.P. (labeled as HS1a and HS1c, respectively), separated by a dry excursion from 16.66 to 16.11kyr B.P. (HS1b). In view of the spatial structure of precipitation anomalies, the widespread increase of monsoon precipitation over the SACZ domain was termed Mega-SACZ. Key Points Reconstruction of the timing and structure of HS1-related SAMS precipitation During HS1 a widespread increase of SAMS rainfall is termed as Mega SACZ SAMS responds almost instantaneously to cooling over North Atlantic.