Device for collecting polluting substances, in particular hydrocarbons, floating on a fluid, in particular water, comprising in combination: a collecting structure for said polluting substances,said collecting structure comprising an inlet zone for a flow of said fluid and of said polluting substances, and an accumulation and retaining zone for the accumulation of said polluting substances; barrier means (140, 141) for diking and directing said polluting substances in said collecting structure, said barrier means (140, 141) extending from said collecting structure in such a manner as to occupy, during operation, at least one region placed above said collecting structure; wall means (112, 114, 115) for conveying and guiding said flow inside said collecting structure; level sensor means (55, 55a, 55b) for detecting the surface level of said fluid and of said polluting substances to determine a thickness of a polluting layer; a processing and control unit able to receive signals indicating one or more operating parameters of said device, and configured for processing said signals for consequently commanding said device; a floating removing mouth positioned at the accumulation and retaining zone and configured for floating on said polluting layer such as to remove selectively only said polluting substances and not said fluid; pump means connected to the floating removing mouth for evacuating the polluting substances; wherein said wall means comprises a first ramp element arranged inside said collecting structure and tilting in a descending manner from said inlet zone towards said accumulating and retaining zone, and a second ramp element located upstream of said first ramp element and intended, during operation, to be tilted in an ascending manner with respect to an advancing direction of said flow. a collecting structure for said polluting substances,said collecting structure comprising an inlet zone for a flow of said fluid and of said polluting substances, and an accumulation and retaining zone for the accumulation of said polluting substances; barrier means (140, 141) for diking and directing said polluting substances in said collecting structure, said barrier means (140, 141) extending from said collecting structure in such a manner as to occupy, during operation, at least one region placed above said collecting structure; wall means (112, 114, 115) for conveying and guiding said flow inside said collecting structure; level sensor means (55, 55a, 55b) for detecting the surface level of said fluid and of said polluting substances to determine a thickness of a polluting layer; a processing and control unit able to receive signals indicating one or more operating parameters of said device, and configured for processing said signals for consequently commanding said device; a floating removing mouth positioned at the accumulation and retaining zone and configured for floating on said polluting layer such as to remove selectively only said polluting substances and not said fluid; pump means connected to the floating removing mouth for evacuating the polluting substances; wherein said wall means comprises a first ramp element arranged inside said collecting structure and tilting in a descending manner from said inlet zone towards said accumulating and retaining zone, and a second ramp element located upstream of said first ramp element and intended, during operation, to be tilted in an ascending manner with respect to an advancing direction of said flow.