The Itajaí-Açu Formation (Portuguese: Formacão Itajaí-Açu) is a geological formation of the Santos Basin offshore of the Brazilian states of Rio de Janeiro, São Paulo, Paraná and Santa Catarina.

The predominantly shale with interbedded turbiditic sandstones formation dates to the Late Cretaceous period; Cenomanian-Maastrichtian epochs and has a maximum thickness of 2,000 metres (6,600 ft).

The Itajaí-Açu Formation is 1,545 to 2,000 metres (5,069 to 6,562 ft) thick,[1] and comprises a thick interval of dark grey clayey rocks, interbedded with the clastics of the Santos and Juréia Formations.

Within this formation, the Ilhabela Member includes the turbiditic sandstones occurring along the section.

Biostratigraphic data from palynomorphs, calcareous nannofossils and planktonic foraminifera indicate a Late Cretaceous age (Cenomanian-Maastrichtian).