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The paper presents the experimental measurements of thermal efficiency of a tubular direct absorption solar collector (DASC) with a hybrid nanofluid based on magnetite (Fe<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula>O<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>4</mn></msub></semantics></math></inline-formula>) and multi-walled carbon nanotubes (MWCNT). The volumetric concentration of Fe<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula>O<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>4</mn></msub></semantics></math></inline-formula> and MWCNT was 0.0053% and 0.0045%, respectively. The experiments were carried out for the flow rates of 2–10 L/min and a temperature difference up to 20 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mo>∘</mo></msup></semantics></math></inline-formula>C between the environment and the DASC. The performance of the DASC with a hybrid nanofluid was in the range of 52.3–69.4%, which was just beyond the performance of the collector with surface absorption. It was also found that using a MWCNT-based nanofluid with an equivalent total volumetric concentration of particles (0.0091%), the efficiency was 8.3–31.5% higher than for the cases with the hybrid nanofluid.