Plants (2.5-)6-15(-25) cm tall. Rhizome erect, short, apex scaly; scales brown to dark brown, triangular to narrowly triangular, 1-3.5 × 0.3-0.7 mm, base fimbriate or entire. Fronds caespitose; stipe (0.7-)1.5-5(-10) cm, green but base castaneous, brown color occasionally extending into rachis, with small dark brown, fimbriate scales and uniseriate hairs, subglabrous when old, adaxially sulcate; lamina triangular to narrowly triangular-ovate, (2-)4-9(-16) × (0.8-)1.2-3(-6.5) cm, base broadly cuneate and slightly reduced, rarely pinnate-pinnatifid, usually bipinnate but occasionally up to 4-pinnatifid, apex acute-acuminate; pinnae (5-)7-10(-15) pairs, alternate or opposite, stalk narrow and up to 3 mm, lower pinnae usually slightly reduced, largest pinnae triangular-ovate, (4-)6-14(-26) × (3-)5-9(-18) mm, rarely up to 45 × 25 mm, base asymmetrical, acroscopic side truncate, basiscopic side cuneate, 1- or 2-pinnate, rarely up to 3-pinnatifid, apex obtuse; pinnules 2-4 pairs (rarely up to 7 pairs), alternate, anadromous, free or adnate, basal acroscopic segment largest and often pinnatifid to almost 2-pinnate in basal part of large fronds, fan-shaped or ovate to triangular-ovate, margin serrate to crenate with acute to mucronate teeth, apex obtuse. Veins obscure, veinlets 1- or 2-forked, not reaching margin. Fronds thinly herbaceous, green to gray-green or brown when dried, rachis green or occasionally with castaneous base (abaxially), adaxially sulcate, subglabrous, lamina with 3- or 4-celled uniseriate gland-tipped hairs, average guard cell length 41-45 µm. Sori 1-3(or 4) per pinnule, median on acroscopic veinlets, occasionally confluent at maturity, oval-linear, (0.8-) 1.1-2(-3) mm; indusia whitish to pale brown, oval-linear, membranous, entire to repand, opening toward major veins or toward costa. Spores with lophate or reticulate perispore, average exospore length 28-32 µm. Plants sexual diploid: 2n = 72.

Asplenium tenuicaule is an ancestral diploid species involved in a reticulate pattern of relationships unraveled in the 1980’s-1990’s (Sleep & Reichstein, Candollea 39: 675-691. 1984; Viane & Reichstein, Pterid. New Millennium, 73-105. 2003) by micromorphological studies and cytological research in China (Y. X. Lin, Z. R. Wang), England (A. Sleep), Germany (H. Rasbach), and Switzerland (T. Reichstein, R. Viane). This reticulate evolution, with its inherent general morphological fuzziness, has led to much misunderstanding and confusion. Asplenium tenuicaule is parental to the following taxa with which it can easily be confused: A. kansuense, A. varians, A. altajense, A. anogrammoides, A. mae, and A. kukkonenii. The name A. varians auct. non Wallich ex Hooker was often used for, or included in, this taxon (e.g., Franchet & Savatier, Enum. Pl. Jap. 220-221. 1877; Beddome, Handb. Ferns Brit. India, 158. 1883; Hillebrand, Fl. Hawaiian Isl. 591. 1888; Hope, J. Bombay Nat. Hist. Soc. 13: 657-671. 1901; Mori, Enum. Pl. Corea, 4. 1922; Ogata, Icon. Fil. Jap. 1: t. 10. 1928; Tardieu, Asplén. Tonkin, 45-46. 1932; Bull. Mus. Natl. Hist. Nat., sér. 2, 5: 480-487. 1933; Ohwi, Fl. Japan, 140. 1957; 95. 1965; Stewart, Biologia 3: 133-164. 1957; Tagawa, Index Pterid. Jap. 179. 1959; Col. Ill. Jap. Pterid. t. 64-343, 152. 1959; Grubov, Rast. Tsentral. Azii 1: 81. 1963; Stewart in Nasir & Ali, Fl. W. Pakistan, 18. 1972; Iwatsuki, Acta Phytotax. Geobot. 25: 69-78. 1972; in Ohashi, Fl. E. Himalaya, 3rd report, 195. 1975; Mitui, Bull. Nippon Dental Coll., Gen. Educ. 4: 221-271. 1975; Nakaike, Enum. Pterid. Jap. 111. 1975; De Vol & Kuo in Li et al., Fl. Taiwan 1: 490. 1975; Kurata & Nakaike, Illustr. Pterid. Japan 2: 226-227. 1981; Ling et al. in Liu et al., Fl. Taiyuan. 41. 1990; in Liu, Fl. Shanxi. 1: 91. 1992; Nakaike & Malik, Cryptog. Fl. Pakistan 1: 270. 1992; Cryptog. Fl. Pakistan 2: 332. 1993). During sporogenesis, plants of this species may produce relatively high numbers of diplospores (a spore containing a double set of chromosomes), which may have played a role in the production of polyploid offspring, e.g., Asplenium kansuense, A. mae (see below).

In places where Asplenium tenuicaule and A. varians grow together, their sterile hybrid is not uncommon.

This is a widespread and variable species ranging from the E African highlands (rare) via the Himalaya to S (Altai) and SE Siberia, Korea, China, Japan, and Hawaii. It can be divided into three varieties, with overlapping ranges in China and Japan. Few but fertile intermediates have been found. Hybridization experiments have shown that the varieties are genetically very similar (Viane & Reichstein, Pterid. New Millennium, 73-105. 2003). Kuo (Taiwania 30: 5-100. 1985) was probably the first to apply the name correctly.

On trees or rocks in forests; 200-3600 m. Chongqing, Gansu, Guizhou, Hebei, Heilongjiang, Henan, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Nei Mongol, Qinghai, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Xizang, Yunnan, Zhejiang [Bhutan, India, Japan, Korea, Nepal, Pakistan, Philippines, Russia (S Siberia), Thailand; E Africa, Pacific islands (Hawaii)].