| 1 | A | B | C | D | E | F | G |
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| 2 | List of undefeated boxing world champions |  |  |  |  |  |  |
| 3 | Boxer | Record | Weight class | Title(s) | Championship years | Title defenses | Notes |
| 4 | United States Jimmy Barry | 59–0–9 (2) | Bantamweight | World | 1894–1899 | 6 (3 draws) | - |
| 5 | United Kingdom Joe Calzaghe | 46–0–0 (0) | Super middleweight | WBO | 1997–2008 | 21 | Vacated title to move up to light heavyweight. |
| 6 | United Kingdom Joe Calzaghe | 46–0–0 (0) | Super middleweight | IBF | 2006 | 1 | Vacated title to fight Peter Manfredo Jr. instead of #1 contender Robert Stieglitz. |
| 7 | United Kingdom Joe Calzaghe | 46–0–0 (0) | Super middleweight | The Ring, lineal | 2006–2008 | 3 | Vacated titles to move up to light heavyweight. |
| 8 | United Kingdom Joe Calzaghe | 46–0–0 (0) | Super middleweight | WBA, WBC | 2007–2008 | 0 | Vacated titles to move up to light heavyweight. |
| 9 | United Kingdom Joe Calzaghe | 46–0–0 (0) | Light heavyweight | The Ring | 2008 | 1 | Retired and vacated title. |
| 10 | South Korea Kim Ji-won | 16–0–2 (0) | Super bantamweight | IBF | 1985–1986 | 4 | - |
| 11 | Romania Mihai Leu | 28–0–0 (0) | Welterweight | WBO | 1997 | 1 | Retired from boxing due to an injury. |
| 12 | Mexico Ricardo López | 51–0–1 (0) | Minimumweight | WBC | 1990–1998 | 22 | Vacated title to move up to light flyweight. |
| 13 | Mexico Ricardo López | 51–0–1 (0) | Minimumweight | WBO | 1997–1998 | 0 | Unified WBC and WBO titles. Stripped of WBO title for saying he wanted to give the belt to his father. |
| 14 | Mexico Ricardo López | 51–0–1 (0) | Minimumweight | WBA | 1998 | 0 | Vacated title to move up to junior flyweight. |
| 15 | Mexico Ricardo López | 51–0–1 (0) | Light flyweight | IBF | 1999–2002 | 2 | - |
| 16 | United States Rocky Marciano | 49–0–0 (0) | Heavyweight | World | 1952–1956 | 6 | - |
| 17 | United Kingdom Terry Marsh | 26–0–1 (0) | Light welterweight | IBF | 1987 | 1 | Retired from boxing due to epilepsy. |
| 18 | United States Floyd Mayweather Jr. | 50–0–0 (0) | Super featherweight | WBC | 1998–2002 | 8 | Vacated title to move up to lightweight. |
| 19 | United States Floyd Mayweather Jr. | 50–0–0 (0) | Lightweight | WBC | 2002–2004 | 3 | Vacated title to move up to light welterweight. |
| 20 | United States Floyd Mayweather Jr. | 50–0–0 (0) | Light welterweight | WBC | 2005–2006 | 0 | Vacated title to move up to welterweight. |
| 21 | United States Floyd Mayweather Jr. | 50–0–0 (0) | Welterweight | WBA, WBC | 2006–2008, 2011–2015 | 8 | Retired at 49–0 in 2015; returned in 2017 for non-title McGregor fight and then retired again. |
| 22 | United States Floyd Mayweather Jr. | 50–0–0 (0) | Light middleweight | WBA, WBC | 2007, 2012–2015, 2017 | 2 | Retired at 49–0 in 2015; returned in 2017 for non-title McGregor fight and then retired again. |
| 23 | Republic of Ireland Jack McAuliffe | 30–0–5 (1) | Lightweight | World | 1886–1893 | 7–0–2 (0) | - |
| 24 | Germany Sven Ottke | 34–0–0 (0) | Super middleweight | IBF | 1998–2004 | 21 | - |
| 25 | Germany Sven Ottke | 34–0–0 (0) | Super middleweight | WBA | 2003–2004 | 4 | Defended IBF title and defeated WBA champion to become WBA Super champion. |
| 26 | Russia Dmitry Pirog | 20–0–0 (0) | Middleweight | WBO | 2010–2012 | 3 | Retired from boxing due to chronic back injuries. |
| 27 | Namibia Harry Simon | 31–0–0 (0) | Light middleweight | WBO | 1998–2001 | 4 | Vacated title to move up to middleweight. |
| 28 | Namibia Harry Simon | 31–0–0 (0) | Middleweight | WBO | 2002 | 0 | Won WBO interim middleweight title in 2001 and the outright title in 2002. Stripped of title at 22–0 when he was unable to defend it due to injuries suffered in a car crash. |
| 29 | Thailand Pichit Sitbangprachan | 24–0–0 (0) | Flyweight | IBF | 1992–1994 | 5 | Retired at 21–0 and then made a comeback. |
| 30 | Venezuela Edwin Valero | 27–0–0 (0) | Super featherweight | WBA | 2006–2008 | 4 | Vacated title to move up to lightweight. |
| 31 | Venezuela Edwin Valero | 27–0–0 (0) | Lightweight | WBC | 2009–2010 | 2 | Vacated title to move up to light welterweight. Committed suicide after allegedly killing his wife in 2010. |
| 32 | United States Andre Ward | 32–0–0 (0) | Super middleweight | WBA | 2009–2015 | 6 | Vacated titles in 2015 to move up to light heavyweight. |
| 33 | United States Andre Ward | 32–0–0 (0) | Super middleweight | WBC, The Ring, lineal | 2011–2015 | 1 (WBC), 2 (The Ring, lineal) | Vacated titles in 2015 to move up to light heavyweight. |
| 34 | United States Andre Ward | 32–0–0 (0) | Light heavyweight | WBA, WBO, The Ring, lineal | 2016–2017 | 1 (WBA, IBF, WBO), 0 (The Ring, lineal) | - |

Question: What is Floyd Mayweather's Jr.'s overall career record, and in how many weight classes did he earn world titles?
Answer: In Floyd Mayweather Jr.'s career, he held multiple world titles in five weight classes (twice at welterweight) and retired with an undefeated record.