Optical probe for measuring, by detection of a light beam originating from at least one radiation source, physical and chemical characteristics of a flowing transparent fluid medium in which the probe is intended to be placed, said probe comprising two rigid fingers (1, 2) spaced apart from each other and having respective longitudinal axes (A 1, A 2) which are parallel, the probe being intended to be oriented so that said longitudinal axes are also parallel to velocity vector of the flow, each finger comprising an end upstream within the flow, an end downstream (16, 26), and a side region of optical quality (13, 23) for transmitting the light beam between said finger and the medium, the downstream end of a first one of the two fingers being adapted to receive the light beam originating from the source, and the downstream end of a second one of the two fingers being adapted to transmit this light beam towards a detector, each upstream end of the fingers (1, 2) having an end face (11, 21) which is oblique relative to the longitudinal axis (A 1, A 2), forming together with a side surface of said finger, a projecting corner in order to split the flow, the side regions of optical quality (13, 23) of the two fingers (1, 2) being situated so as to transmit the light beam from one to the other of said regions through the medium, in a path between the two fingers which is not orthogonal to the flow velocity vector, the probe being characterized by each finger (1, 2) comprising a solid rod that is single piece out of a material optically transparent to the light beam, the rod extending from the downstream end (16, 26) to the upstream end face (11, 21) of said finger, and the side region of optical quality (13, 23) of each finger being formed by a refracting surface between the material of the rod and the medium, the upstream end face (11, 21) of each finger (1, 2) being of optical quality, and the side region of optical quality (13, 23) of each finger (1, 2) being situated in the projecting corner of said finger, and by the probe being arranged so that, during use of the probe, the light beam received at the downstream end of the first finger is propagated rectilinearly and longitudinally in the rod of the first finger, inside the material of said rod, then is reflected on the upstream end face of the first finger inside the rod of said first finger, towards the side region of optical quality of the first finger, then is refracted by the refracting surface of the side region of optical quality of the first finger, from the material of the rod of said first finger to the medium, towards the side region of optical quality of the second finger, then traverses the medium between the first and second fingers, then is refracted by the refracting surface of the side region of optical quality of the second finger, from the medium to the material of the rod of the said second finger, in the direction of the upstream end face of the second finger, then is reflected on the upstream end face of the second finger inside the rod of said second finger, then is propagated rectilinearly and longitudinally within the rod of the second finger, inside the material of said rod, towards the downstream end of the second finger, and by the probe being adapted so that the reflections of the light beam on the respective upstream end faces (11, 21) of the two fingers (1, 2) are total reflections, the upstream end face (11, 21) of at least one of the fingers (1, 2) being a dioptric interface between the material of the corresponding rod and the medium, and the reflection of the light beam on said upstream end face being a total internal reflection, with an angle of incidence (i 1, i 7) of said light beam relative to a direction perpendicular to said upstream end face which is greater than a critical angle transmission value given by Snell's law.