Context : Super-luminous supernovae ( SLSNe ) are rare events defined as being significantly more luminous than normal terminal stellar explosions .
The source of the additional power needed to achieve such luminosities is still unclear .
Discoveries in the local Universe ( i.e . z < 0.1 ) are scarce , but afford dense multi-wavelength observations .
Additional low-redshift objects are therefore extremely valuable .

Aims : We present early-time observations of the type I SLSN ASASSN-18km/SN 2018bsz .
These data are used to characterise the event and compare to literature SLSNe and spectral models .
Host galaxy properties are also analysed .

Methods : Optical and near-IR photometry and spectroscopy were analysed .
Early-time ATLAS photometry was used to constrain the rising light curve .
We identified a number of spectral features in optical-wavelength spectra and track their time evolution .
Finally , we used archival host galaxy photometry together with H ii region spectra to constrain the host environment .

Results : ASASSN-18km/SN 2018bsz is found to be a type I SLSN in a galaxy at a redshift of 0.0267 ( 111 Mpc ) , making it the lowest-redshift event discovered to date .
Strong C ii lines are identified in the spectra .
Spectral models produced by exploding a Wolf-Rayet progenitor and injecting a magnetar power source are shown to be qualitatively similar to ASASSN-18km/SN 2018bsz , contrary to most SLSNe-I that display weak or non-existent C ii lines .
ASASSN-18km/SN 2018bsz displays a long , slowly rising , red ‘ plateau ’ of > 26 days , before a steeper , faster rise to maximum .
The host has an absolute magnitude of –19.8 mag ( r ) , a mass of M _ { * } = 1.5 ^ { +0.08 } _ { -0.33 } \times 10 ^ { 9 } M _ { \odot } , and a star formation rate of = 0.50 ^ { +2.22 } _ { -0.19 } M _ { \odot } yr ^ { -1 } .
A nearby H ii region has an oxygen abundance ( O3N2 ) of 8.31 \pm 0.01 dex .

Conclusions :