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Road pavement characteristics can be improved by modifying bitumens through addition of fine particles. To avoid environmental issues, attention is recently being paid to bio-materials. In this study, a 50/70 bitumen was modified through the addition of char obtained from the pyrolysis of waste tires. Char addition causes an increase in transition (gel to sol) temperature of up to 4 °C and an increase in rigidity under working conditions (50 °C) of up to about one order of magnitude. The effect of the gas type flowing under the pyrolysis process (CO₂, N₂ and CO₂ + N₂) on the mechanical characteristics of the bitumen was also investigated. More marked effects on the rheological performances were found if char coming from pyrolysis under CO₂ was used (the gel-to-sol transition temperature is increased by about 4.5 °C) compared to that coming from pyrolysis under N₂ (gel-to-sol transition temperature is increased by about 3 °C). The effect is at a maximum for char from CO₂ at 3% wt/wt, whereas regarding char from N, a lesser effect is seen with a more uniform (flat) effect as a function of char% and with an increase in gel-to-sol transition temperature of about 3 °C. Coherently, char obtained from pyrolysis under CO₂ was found to possess a higher surface area constituted by smaller particles than that coming from pyrolysis under N₂. In fact, the BET-specific surface passes 79 m²/g for N₂-char to 174 m²/g for CO₂-char and the micropore volume fraction increases from 2 to 9. The observed differences can be attributed to the oxidizing environment (CO₂) that is more aggressive and reactive in the synthesis phase compared to an inert environment (N₂). Char also showed an anti-aging effect, hindering the increase in rigidity typically associated with the aging process. This effect was explained in terms of the compatibility of char with bitumen’s organic nature, and presumably its more effective hosting in a bituminous structure, which is presumably higher when char is produced under CO₂ rather than N₂. This study quantitatively indicates how a residue derived from the pyrolysis of a waste material can be efficiently re-used to increase the mechanical characteristics of bitumen, accomplishing the recent circular-based needs for environmental protection.