Sellas Life Sciences (SLS) had the highest return in December 2025 by a mid-cap US stock, returning 138.6%.

ASSET	MONTH	% RETURN
Sellas Life Sciences (SLS)	December 2025	138.61%
Structure Therapeutics Inc American Depositary Shares (GPCR)	December 2025	97.25%
Hycroft Mining (HYMC)	December 2025	91.93%
ChronoScale (CHRN)	December 2025	88.24%
Regencell Bioscience (RGC)	December 2025	82.45%
Intuitive Machines (LUNR)	December 2025	76.22%
Planet Labs PBC (PL)	December 2025	70%
Andersen (ANDG)	December 2025	62.06%
Digitalbridge (DBRG)	December 2025	61.41%
AXT (AXTI)	December 2025	55.57%
Vera Therapeutics (VERA)	December 2025	51.75%
Redwire (RDW)	December 2025	43.13%
ADEIA (ADEA)	December 2025	41.51%
Maase (MAAS)	December 2025	36.71%
Century Aluminum (CENX)	December 2025	31.21%
Alumis (ALMS)	December 2025	30.48%
Firefly Aerospace Inc Common Stock (FLY)	December 2025	30.13%
American Eagle Outfitters (AEO)	December 2025	29.71%
Abercrombie & Fitch (ANF)	December 2025	29.54%
Almonty Industries Inc Common Shares (ALM)	December 2025	28.61%
Polestar Automotive Holding UK PLC Class A ADS (PSNY)	December 2025	27.66%
Butterfly Network (BFLY)	December 2025	27.52%
Middleby (MIDD)	December 2025	26.86%
Aura Minerals Inc Common Shares (AUGO)	December 2025	26.35%
Kodiak Sciences (KOD)	December 2025	25.83%

Mid-cap stocks are treated as having a market cap between $2B and $10B.