**Richard Gordon Thomson leads the 2026 Aberdeen South by-election market at 71.5% implied probability as the SNP candidate, while Douglas Lumsden trails at 27% for the Conservatives and all other named contenders sit at 50% or lower.** The by-election, scheduled for 18 June 2026, was triggered when SNP MP Stephen Flynn resigned on 14 May after winning a Scottish Parliament seat, following the Scottish Elections (Representation and Reform) Act 2025 ban on dual mandates. Thomson, a former MP for the abolished Gordon constituency with deep local roots and prior Aberdeenshire Council leadership, benefits from the SNP’s established position in the seat, which Flynn held since 2019. Lumsden, a sitting North East Scotland MSP and prior Conservative candidate in the area, positions his campaign around energy security and North Sea oil-and-gas policy amid ongoing voter concerns in Aberdeen’s energy sector. Minor-party and independent candidates face structural barriers under first-past-the-post, limiting their realistic paths. Traders appear to price the contest as a two-way race shaped by recent Scottish Parliament results, candidate familiarity, and the short campaign timeline, with no major late developments reported that have shifted the consensus in the final days before polling.