Altimmune (ALT) had the highest return in May 2020 by a small-cap US stock, returning 205.3%.

ASSET	MONTH	% RETURN
Altimmune (ALT)	May 2020	205.26%
Akso Health Group ADR (AHG)	May 2020	197.37%
Nabors Industries (NBR)	May 2020	168.16%
Novavax (NVAX)	May 2020	158.65%
Camping World (CWH)	May 2020	148.59%
International Tower Hill Mines (THM)	May 2020	145.65%
Babcock & Wilcox Enterprises (BW)	May 2020	138.89%
Northern Dynasty Minerals (NAK)	May 2020	130.09%
Arbutus Biopharma (ABUS)	May 2020	110.68%
MMTEC (MTC)	May 2020	108.82%
CPI Card (PMTS)	May 2020	104.41%
Kopin (KOPN)	May 2020	103.92%
Xeris Pharmaceuticals (XERS)	May 2020	91.79%
Fiverr International (FVRR)	May 2020	74.82%
Avino Silver & Gold Mines (ASM)	May 2020	74.68%
Dauch (DCH)	May 2020	73.41%
Allogene Therapeutics (ALLO)	May 2020	70.96%
Sprout Social (SPT)	May 2020	68.5%
Tanzanian Royalty Exploration (TRX)	May 2020	64.75%
Village Farms International (VFF)	May 2020	64.35%
Everspin Technologies (MRAM)	May 2020	63.16%
Obsidian Energy (OBE)	May 2020	61.9%
Tectonic Therapeutic (TECX)	May 2020	59.84%
Upwork (UPWK)	May 2020	55.11%
Lincoln Educational Services (LINC)	May 2020	54.15%

Small-cap stocks are treated as having a market cap between $300M and $2B.