Maximilian Adelbert Baer Sr. (February 11, 1909 – November 21, 1959) was an American professional boxer and the world heavyweight champion from June 14, 1934, to June 13, 1935. He was known in his time as the Livermore Larupper and Madcap Maxie. Two of his fights (a 1933 win over Max Schmeling and a 1935 loss to James J. Braddock) were rated Fight of the Year by The Ring magazine. Baer was also a boxing referee, and had occasional roles in film and television. He was the brother of heavyweight boxing contender Buddy Baer and father of actor Max Baer Jr. Baer is rated #22 on The Ring magazine's list of 100 greatest punchers of all time.

Baer was born on February 11, 1909, in Omaha, Nebraska, to Jacob Baer (1875–1938) and Dora Bales (1877–1938). His father was the son of Jewish immigrants from Alsace-Lorraine and his mother was of Scottish descent. His elder sister was Frances May Baer (1905–1991), his younger sister was Bernice Jeanette Baer (1911–1987), his younger brother was boxer-turned-actor Jacob Henry Baer, better known as Buddy Baer (1915–1986), and his adopted brother was August "Augie" Baer.

Baer moved to Livermore, California, with his family as a child. He attended high school only one year, quitting to work in his father's butcher shop. Livermore was ranch and cowboy country, surrounded by tens of thousands of acres of open land on which large cattle herds grazed and provided fresh meat to the local area. In 1928, Jacob leased the Twin Oaks Ranch in Murray Township, where he raised more than 2,000 hogs and worked with daughter Frances' husband, Louis Santucci. Baer often credited his work carrying heavy carcasses of meat, stunning cattle with one blow, and working at a gravel pit for the development of his powerful shoulders (an article in the January 1939 edition of Family Circle magazine reported that Baer also took the Charles Atlas exercise course.) By age eighteen, Baer stood 6 feet tall and weighed 190 pounds.

Baer turned professional in 1929, progressing steadily through the Pacific Coast ranks. His career was not without setbacks; in 1930 he lost a decision to seasoned heavyweight Les Kennedy in Los Angeles. Even so, Baer built a strong record and scored victories over several more experienced fighters. His growing popularity stemmed largely from his aggressive fighting style. Although he lacked refined technical skills, he compensated with relentless offense, repeatedly charging his opponents and wearing them down with powerful punches. As a result, many of his matches ended in knockouts.

Baer fought Frankie Campbell on August 25, 1930, at San Francisco's Recreation Park for the unofficial title of Pacific Coast champion. In the second round, Campbell clipped Baer and Baer slipped to the canvas. Campbell went toward his corner and waved to the crowd, thinking that Baer was getting the count. In response, Baer got up and flew at Campbell, landing a right to Campbell's turned head which sent him to the canvas.

After the round, Campbell said to his trainer "Something feels like it snapped in my head", but he went on to handily win rounds 3 and 4. As Baer rose for the 5th round, Tillie "Kid" Herman, Baer's former friend and trainer, who had switched camps overnight and was now in Campbell's corner, savagely taunted and jeered Baer. In a rage and determined to end the bout with a knockout, Baer soon had Campbell against the ropes. As he hammered him with punch after punch, the ropes were the only thing holding Campbell up. By the time referee Toby Irwin stopped the fight, Campbell collapsed to the canvas. Baer's own seconds reportedly ministered to Campbell, and Baer stayed by his side until an ambulance arrived 30 minutes later. Baer "visited the stricken fighter's bedside", where he offered Frankie's wife Ellie the hand that hit her husband. She took that hand and the two stood speechless for a moment. "It was unfortunate, I'm awfully sorry", said Baer. "It could have been you," she replied. She forgave him.

At noon the next day, with a lit candle laced between his crossed fingers, and his wife and mother beside him, Frankie Campbell was pronounced dead. Upon the surgeon's announcement of Campbell's death, Baer broke down and sobbed inconsolably. Brain specialist Dr. Tilton E. Tillman "declared death had been caused by a succession of blows on the jaw and not by any struck on the rear of the head" and that Campbell's brain had been "knocked completely loose from his skull" by Baer's blows.

In 1930, Baer was charged with manslaughter. He was ultimately cleared of criminal charges, but the California State Athletic Commission suspended him from boxing in the state for one year. Following Campbell's death, Baer stepped away from boxing for several months. Upon returning, he lost four of his next six bouts, in part due to a hesitancy to press the attack. One of those victories came at the hands of future Hall of Famer Tommy Loughran, who observed that Baer was looping and telegraphing his punches. Former heavyweight champion Jack Dempsey later worked with Baer to shorten his punches and remained interested in his development throughout the rest of his career.