Caleb Boushley had the highest K/9 by a relief pitcher in 2023, with 19.29 per nine innings in his rookie season.

	NAME	K/9	SEASON	TM	G	W	L	W%	GS	ERA	SO	CG	SHO	SV	IP	H	ER	R	HR	BB	HBP	GF	IBB	TBF	WP	BB/9	K/BB	HR/9	K%	BB%	WHIP	BABIP	RA	H/9
1	Caleb Boushley C. Boushley	19.29	2023	MIL	1	1	0	1.000	0	3.86	5	0	0	0	2.1	1	1	1	1	2	0	1	1	10	1	7.71	2.50	3.86	50.0	20.0	1.29	.000	3.86	3.86
2	Sean Reid-Foley S. Reid-Foley	18.78	2023	NYM	8	0	1	.000	0	3.52	16	0	0	0	7.2	4	3	3	0	6	0	1	0	34	0	7.04	2.67	0.00	47.1	17.6	1.30	.333	3.52	4.70
3	Yoendrys Gómez Y. Gómez	18.00	2023	NYY	1	0	0		0	0.00	4	0	0	0	2.0	1	0	0	0	0	1	1	0	8	0	0.00		0.00	50.0	0.0	0.50	.333	0.00	4.50
4	Jose Espada J. Espada	18.00	2023	SDP	1	0	0		0	0.00	2	0	0	0	1.0	0	0	0	0	2	0	1	0	5	0	18.00	1.00	0.00	40.0	40.0	2.00	.000	0.00	0.00
5	Orion Kerkering O. Kerkering	18.00	2023	PHI	3	1	0	1.000	0	3.00	6	0	0	0	3.0	3	1	1	0	2	0	0	0	14	0	6.00	3.00	0.00	42.9	14.3	1.67	.500	3.00	9.00
6	John McMillon J. McMillon	18.00	2023	KCR	4	0	0		0	2.25	8	0	0	0	4.0	1	1	1	1	0	0	3	0	13	0	0.00		2.25	61.5	0.0	0.25	.000	2.25	2.25
7	Ryder Ryan R. Ryan	18.00	2023	SEA	1	0	0		0	0.00	2	0	0	0	1.0	0	0	0	0	1	0	0	0	4	0	9.00	2.00	0.00	50.0	25.0	1.00	.000	0.00	0.00
8	Alex Speas A. Speas	18.00	2023	TEX	3	0	2	.000	0	13.50	4	0	0	0	2.0	2	3	3	0	5	0	1	1	12	1	22.50	0.80	0.00	33.3	41.7	3.50	.667	13.50	9.00
9	Guillo Zuñiga G. Zuñiga	18.00	2023	STL	2	0	0		0	4.50	4	0	0	0	2.0	2	1	1	0	0	0	2	0	8	0	0.00		0.00	50.0	0.0	1.00	.500	4.50	9.00
10	Darren McCaughan D. McCaughan	18.00	2023	SEA	3	0	0		0	5.40	10	0	0	0	5.0	7	3	5	1	3	0	3	0	27	0	5.40	3.33	1.80	37.0	11.1	2.00	.462	9.00	12.60
11	Félix Bautista F. Bautista	16.23	2023	BAL	56	8	2	.800	0	1.48	110	0	0	33	61.0	30	10	14	4	26	2	46	0	237	7	3.84	4.23	0.59	46.4	11.0	0.92	.274	2.07	4.43
12	Luke Little L. Little	16.20	2023	CHC	7	0	0		0	0.00	12	0	0	0	6.2	5	0	0	0	4	1	3	0	30	1	5.40	3.00	0.00	40.0	13.3	1.35	.385	0.00	6.75
13	Prelander Berroa P. Berroa	16.20	2023	SEA	2	0	0		0	0.00	3	0	0	0	1.2	0	0	0	0	3	0	0	0	8	1	16.20	1.00	0.00	37.5	37.5	1.80	.000	0.00	0.00
14	Aroldis Chapman A. Chapman	15.89	2023	TOT	61	6	4	.600	0	3.09	103	0	0	6	58.1	37	20	24	4	36	1	18	2	249	9	5.55	2.86	0.62	41.4	14.5	1.25	.314	3.70	5.71
15	Nick Hernandez N. Hernandez	15.00	2023	SDP	2	0	0		0	12.00	5	0	0	0	3.0	3	4	4	1	4	2	0	0	18	0	12.00	1.25	3.00	27.8	22.2	2.33	.333	12.00	9.00
16	Alek Jacob A. Jacob	15.00	2023	SDP	3	0	0		0	0.00	5	0	0	0	3.0	0	0	0	0	1	0	2	0	10	0	3.00	5.00	0.00	50.0	10.0	0.33	.000	0.00	0.00
17	Daniel Hudson D. Hudson	15.00	2023	LAD	3	0	0		0	0.00	5	0	0	1	3.0	2	0	0	0	3	0	1	0	14	0	9.00	1.67	0.00	35.7	21.4	1.67	.333	0.00	6.00
18	Daysbel Hernández D. Hernández	14.73	2023	ATL	4	1	0	1.000	0	7.36	6	0	0	0	3.2	6	3	3	1	3	0	2	0	20	0	7.36	2.00	2.45	30.0	15.0	2.45	.500	7.36	14.73
19	Chris Paddack C. Paddack	14.40	2023	MIN	2	1	0	1.000	0	5.40	8	0	0	0	5.0	6	3	3	1	1	0	0	0	22	1	1.80	8.00	1.80	36.4	4.5	1.40	.417	5.40	10.80
20	Shawn Dubin S. Dubin	14.40	2023	HOU	2	0	0		0	10.80	8	0	0	0	5.0	9	6	6	1	1	0	1	0	25	0	1.80	8.00	1.80	32.0	4.0	2.00	.533	10.80	16.20
21	Kody Funderburk K. Funderburk	14.25	2023	MIN	11	2	0	1.000	0	0.75	19	0	0	0	12.0	6	1	1	1	5	1	4	0	47	0	3.75	3.80	0.75	40.4	10.6	0.92	.238	0.75	4.50
22	José Alvarado J. Alvarado	13.94	2023	PHI	42	0	2	.000	0	1.74	64	0	0	10	41.1	30	8	15	3	18	0	12	1	172	3	3.92	3.56	0.65	37.2	10.5	1.16	.310	3.27	6.53
23	Joe Kelly J. Kelly	13.73	2023	TOT	42	2	5	.286	0	4.12	60	0	0	1	39.1	29	18	22	3	18	2	6	0	168	6	4.12	3.33	0.69	35.7	10.7	1.19	.306	5.03	6.64
24	Angel Perdomo A. Perdomo	13.66	2023	PIT	30	3	2	.600	0	3.72	44	0	0	0	29.0	21	12	14	3	11	3	8	3	117	0	3.41	4.00	0.93	37.6	9.4	1.10	.321	4.34	6.52
25	Matt Brash M. Brash	13.63	2023	SEA	78	9	4	.692	0	3.06	107	0	0	4	70.2	65	24	26	3	29	6	7	0	308	11	3.69	3.69	0.38	34.7	9.4	1.33	.380	3.31	8.28