Sea spiders are marine arthropods of the class Pycnogonida, hence they are also called pycnogonids (/pɪkˈnɒɡənədz/; named after Pycnogonum, the type genus; with the suffix -id). The class includes the only extant order Pantopoda (lit. 'all feet'), alongside a few fossil species which could trace back to the early or mid-Paleozoic. They are cosmopolitan, found in oceans around the world. The over 1,300 known species have leg spans ranging from 1 mm (0.04 in) to over 70 cm (2.3 ft). Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in Antarctic and deep waters.

Despite their name and slight resemblance, "sea spiders" are not spiders, nor even arachnids. While some literature around the 2000s suggests they may be a sister group to all other living arthropods, their traditional classification as a member of chelicerates alongside horseshoe crabs and arachnids has regained wide support in subsequent studies.

Many sea spiders are recognised by their enormous walking legs in contrast to a reduced body region, resulting into the so-called "all legs" or "no body" appearance. The body segments (somites) are generally interpreted as three main sections (tagma): cephalon (head, aka cephalosoma), trunk (aka thorax) and abdomen. However, the definition of cephalon and trunk might differ between literature (see text), and some studies might follow a prosoma (=cephalon+trunk)–opisthosoma (=abdomen) definition, aligning to the tagmosis of other chelicerates. The exoskeleton of the body is tube-like, lacking the dorsoventral division (tergite and sternite) seen in most other arthropods.

The cephalon is formed by the fusion of ocular somite and four anterior segments behind it (somite 1–4). It consists of an anterior proboscis, a dorsal ocular tubercle with eyes, and up to four pairs of appendages (chelifores, palps, ovigers and first walking legs). Although some literature might consider the segment carrying the first walking leg (somite 4) to be part of the trunk, it is completely fused to the remaining head section to form a single cephalic tagma. The proboscis has three-fold symmetry, terminating with a typically Y-shaped mouth (vertical slit in Austrodecidae). It usually has fairly limited dorsoventral and lateral movement. However, in those species that have reduced chelifores and palps, the proboscis is well developed and flexible, often equipped with numerous sensory bristles and strong rasping ridges around the mouth. The proboscis is unique to pycnogonids, and its exact homology with other arthropod mouthparts is enigmatic, as well as its relationship with the absence of labrum (preoral upper lip of ocular somite) in pycnogonid itself. The ocular tubercle has up to two pairs of simple eyes (ocelli) on it, though sometimes the eyes can be reduced or missing, especially among species living in the deep oceans. All of the eyes are median eyes in origin, homologous to the median ocelli of other arthropods, while the lateral eyes (e.g. compound eyes) found in most other arthropods are completely absent.

In adult pycnogonids, the chelifores (aka cheliphore), palps and ovigers (aka ovigerous legs) are variably reduced or absent, depending on taxa and sometimes sex. Nymphonidae is the only family where all of three pairs are always functional. The ovigers can be reduced or missing in females, but are present in almost all males. In a functional condition, the chelifores terminate with a pincer (chela) formed by two segments (podomeres), like the chelicerae of most other chelicerates. The scape (peduncle) behind the pincer is usually unsegmented, but could be bisegmented in some species, resulting into a total of three or four chelifore segments. The palps and ovigers have up to 9 and 10 segments respectively, but can have fewer even when in a functional condition. The palps are rather featureless and never have claws in adult Pantopoda, while the ovigers may or may not possess a terminal claw and rows of specialised spines on its curved distal segments (strigilis). The chelifores are used for feeding and the palps are used for sensing and manipulating food items, while the ovigers are used for cleaning themselves, with the additional function of carrying offspring in males.

The leg-bearing somites (somite 4 and all trunk somites, the alternatively defined "trunk/thorax") are either segmented or fused to each other, carrying the walking legs via a series of lateral processes (lateral tubular extension of the somites). In most species, the legs are much larger than the body in both length and volume, only being shorter and more slender than the body in Rhynchothoracidae. Each leg is typically composed of eight tubular segments, commonly known as coxa 1, 2 and 3, femur, tibia 1 and 2, tarsus, and propodus. This terminology, with three coxae, no trochanter, and using the term "propodus", is unusual for arthropods. However, based on muscular system and serial homology to the podomeres of other chelicerates, they are most likely coxa (=coxa 1), trochanter (=coxa 2), prefemur/basifemur (=coxa 3), postfemur/telofemur (=femur), patella (=tibia 1), tibia (tibia 2) and two tarsomeres (=tarsus and propodus) in origin. The leg segmentation of Paleozoic taxa is a bit different, noticeably they have annulated coxa 1 and are further divided into two types: one with flattened distal (femur and beyond) segments and first leg pair with one less segment than the other leg pairs (e.g. Palaeoisopus, Haliestes), and another one with an immobile joint between the apparently fourth and fifth segment which altogether might represent a divided femur (e.g. Palaeopantopus, Flagellopantopus). Each leg terminates with a main claw (aka pretarsus/apotele, the true terminal segment), which may or may not have a pair of auxiliary claws on its base. Most of the joints move vertically, except the joint between coxa 1–2 (coxa-trochanter joint) which provide lateral mobility (promotor-remotor motion), and the joint between tarsus and propodus did not have muscles, just like the subdivided tarsus of other arthropods. Adults usually have eight legs (four pairs) in total, but in a few species, adults have five to six pairs. These are known as polymerous (i.e., extra-legged) species, which are distributed among six genera in the families Pycnogonidae (five pairs in Pentapycnon), Colossendeidae (five pairs in Decolopoda and Pentacolossendeis, six pairs in Dodecolopoda) and Nymphonidae (five pairs in Pentanymphon, six pairs in Sexanymphon).

Several alternatives had been proposed for the position homology of pycnogonid appendages, such as chelifores being protocerebral/homologous to the labrum (see text) or ovigers being duplicated palps. Conclusively, the classic, morphology-based one-by-one alignment to the prosomal appendages of other chelicerates was confirmed by both neuroanatomic and genetic evidences. Noticeably, the order of pycnogonid leg pairs are mismatched to those of other chelicerates, starting from the ovigers which are homologous to the first leg pair of arachnids. While the fourth walking leg pair was considered aligned to the variably reduced first opisthosomal segment (somite 7, also counted as part of the prosoma based on different studies and/or taxa) of euchelicerates, the origin of the additional fifth and sixth leg pairs in the polymerous species are still enigmatic. Together with the cephalic position of the first walking legs, the anterior and posterior boundary of pycnogonid leg pairs are not aligned to those of euchelicerate prosoma and opisthosoma, nor the cephalon and trunk of pycnogonid itself.

The abdomen (aka trunk end) does not have any appendages. In Pantopoda it is also called the anal tubercle, which is always unsegmented, highly reduced and almost vestigial, simply terminated by the anus. It is considered to be a remnant of opisthosoma/trunk of other chelicerates, but it is unknown which somite (s) it actually aligned to. So far only Paleozoic species have segmented abdomens (at least up to four segments, presumably somite 8–11 which aligned to opisthosomal segment 2–5 of euchelicerates), with some of them even terminated by a long telson (tail).