Company: SNY
Filing Date: 2025-10-29
Form Type: 424B5
Source: 0001193125-25-255563
Chunk: 2

Company: Sanofi
Filing Date: 2025-10-29
Form: 424B5
Chunk 2
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 $2,000 and integral multiples of $1,000.
See “Description of the Notes.”

The notes will be our unsecured and unsubordinated obligations, and therefore will rank equally with each
other and with all of our existing and future unsecured and unsubordinated debt obligations.

We do not intend to list the notes on any securities exchange or automated quotation system. Investing in the notes involves risks. Prior to making a decision about investing in the notes, you should carefully consider the specific factors that are described in the “ Risk Factors” section beginning on page S-14of this prospectus supplement and page 7 of the attached prospectus.

| Per 2027 Floating Rate Note |     | Price to Public (1) |       100.000 | % |     | Underwriting 
 Discounts    |     0.200 | % |     | Proceeds, Before 
 Expenses, to     
 Sanofi (1)       |        99.800 | % |
|:----------------------------|:----|:--------------------|--------------:|:--|:----|:-------------|----------:|:--|:----|:-----------------|--------------:|:--|
| Total                       |     | $                   |   500,000,000 |   |     | $            | 1,000,000 |   |     | $                |   449,000,000 |   |
| Per 2028 Floating Rate Note |     |                     |       100.000 | % |     |              |     0.250 | % |     |                  |        99.750 | % |
| Total                       |     | $                   |   500,000,000 |   |     | $            | 1,250,000 |   |     | $                |   498,750,000 |   |
| Per 2027 Fixed Rate Note    |     |                     |        99.998 | % |     |              |     0.200 | % |     |                  |        99.798 | % |
| Total                       |     | $                   |   399,992,000 |   |     | $            |   800,000 |   |     | $                |   399,192,000 |   |
| Per 2028 Fixed Rate Note    |     |                     |        99.994 | % |     |              |     0.250 | % |     |                  |