Towns are marked in bold and market towns in italics: Bělá - Benecko - Benešov u Semil - Bozkov - Bradlecká Lhota - Bukovina u Čisté - Bystrá nad Jizerou - Chuchelna - Čistá u Horek - Háje nad Jizerou - Holenice - Horka u Staré Paky - Horní Branná - Hrubá Skála - Jablonec nad Jizerou - Jesenný - Jestřabí v Krkonoších - Jilemnice - Kacanovy - Karlovice - Klokočí - Košťálov - Kruh - Ktová - Levínská Olešnice - Libštát - Lomnice nad Popelkou - Loučky - Martinice v Krkonoších - Mírová pod Kozákovem - Modřišice - Mříčná - Nová Ves nad Popelkou - Ohrazenice - Olešnice - Paseky nad Jizerou - Peřimov - Poniklá - Přepeře - Příkrý - Radostná pod Kozákovem - Rakousy - Rokytnice nad Jizerou - Roprachtice - Rovensko pod Troskami - Roztoky u Jilemnice - Roztoky u Semil - Semily - Slaná - Stružinec - Studenec - Svojek - Syřenov - Tatobity - Troskovice - Turnov - Veselá - Víchová nad Jizerou - Vítkovice - Všeň - Vyskeř - Vysoké nad Jizerou - Záhoří - Žernov Semily District briefly borders Poland in the north.

The terrain is very diverse, mountainous in the north, foothills in the middle and slightly undulating in the south.

The territory extends into five geomorphological mesoregions: Giant Mountains Foothills (most of the territory), Giant Mountains (north), Ještěd–Kozákov Ridge (a strip from west to south) and Jičín Uplands (southwest).

The lowest point is the river bed of the Jizera in Všeň at 236 m (774 ft).

In the west, two separate parts of the Bohemian Paradise Protected Landscape Area lie within the district.