A molecular Bose-Einstein condensate (BEC) was directly observed in an ultracold Fermi gas of atoms by tuning the interaction strength across a Feshbach resonance. This state of matter represents one extreme of the BCS-BEC crossover. The experiment involved starting with a quantum-degenerate Fermi gas and adiabatically creating molecules through a magnetic-field sweep across a Feshbach resonance. The resulting molecular sample exhibited a significant condensate fraction, confirmed by time-of-flight absorption images. The molecular BEC was not formed by active cooling but by traversing the BCS-BEC crossover regime. The molecules formed were weakly bound, highly vibrationally excited, and extremely large in spatial extent. The experiment demonstrated that the molecular BEC could be created by adjusting the interaction strength in the ultracold Fermi gas. The results showed a bimodal momentum distribution, indicating a phase transition to a BEC. The condensate fraction was measured as a function of temperature, revealing the onset of Bose-Einstein condensation at a temperature of ~0.8Tc. The critical temperature for the interacting molecules was found to be 0.8±0.1Tc, lower than the ideal gas prediction due to repulsive interactions. The creation of the molecular BEC required a slow traversal of the Feshbach resonance, and the condensate fraction was found to depend on the ramp time across the resonance. The experiment also showed that the molecular BEC is related continuously to BCS-type superfluidity on the attractive side of the resonance. The results provide insights into the BCS-BEC crossover and the behavior of fermionic superfluidity in ultracold gases.A molecular Bose-Einstein condensate (BEC) was directly observed in an ultracold Fermi gas of atoms by tuning the interaction strength across a Feshbach resonance. This state of matter represents one extreme of the BCS-BEC crossover. The experiment involved starting with a quantum-degenerate Fermi gas and adiabatically creating molecules through a magnetic-field sweep across a Feshbach resonance. The resulting molecular sample exhibited a significant condensate fraction, confirmed by time-of-flight absorption images. The molecular BEC was not formed by active cooling but by traversing the BCS-BEC crossover regime. The molecules formed were weakly bound, highly vibrationally excited, and extremely large in spatial extent. The experiment demonstrated that the molecular BEC could be created by adjusting the interaction strength in the ultracold Fermi gas. The results showed a bimodal momentum distribution, indicating a phase transition to a BEC. The condensate fraction was measured as a function of temperature, revealing the onset of Bose-Einstein condensation at a temperature of ~0.8Tc. The critical temperature for the interacting molecules was found to be 0.8±0.1Tc, lower than the ideal gas prediction due to repulsive interactions. The creation of the molecular BEC required a slow traversal of the Feshbach resonance, and the condensate fraction was found to depend on the ramp time across the resonance. The experiment also showed that the molecular BEC is related continuously to BCS-type superfluidity on the attractive side of the resonance. The results provide insights into the BCS-BEC crossover and the behavior of fermionic superfluidity in ultracold gases.