The invention relates to a breathing device (1), comprising a mouthpiece (2) forming a breathing channel to form a connection between a first (3) end and a second (4) end of the mouthpiece (2). The first (3) end is configured for a user breathing into the mouthpiece through a breathing opening (5). An at least partly flexible rebreathing air chamber (6) is arranged in fluid communication with the second (4) end of the mouthpiece, thereby being in fluid connection with the breathing channel through the second end of the mouthpiece (2). The rebreathing air chamber is formed by at least partly flexible wall section(s). A wall member (7) is arranged at the second (4) end of the mouthpiece and/or attached to the rebreathing air chamber (6). The wall member (7) comprises one or more through-going openings (8) provided in the wall member (7) and/or in the breathing channel, allowing fluid communication between the rebreathing air chamber (6) and the surrounding atmosphere.
