Aroldis Chapman has the highest K/9 with 50.0+ innings pitched in a season, with 17.67 per nine innings in 2014.

Interpreted as:

highest k/9 by a player in season min 50 ip

	NAME	K/9	IP	SEASON	TM	G	W	L	W%	GS	ERA	SO	CG	SHO	SV	H	ER	R	HR	BB	HBP	GF	IBB	TBF	WP	BB/9	K/BB	HR/9	K%	BB%	WHIP	BABIP	RA	ERA-	FIP	FIP-	H/9	PRAA
1	Aroldis Chapman A. Chapman	17.67	54.0	2014	CIN	54	0	3	.000	0	2.00	106	0	0	36	21	12	12	1	24	2	44	0	202	4	4.00	4.42	0.17	52.5	11.9	0.83	.290	2.00	54	0.89	23	3.50	17
2	Edwin Díaz E. Díaz	17.13	62.0	2022	NYM	61	3	1	.750	0	1.31	118	0	0	32	34	9	9	3	18	2	49	1	235	2	2.61	6.56	0.44	50.2	7.7	0.84	.330	1.31	34	0.90	23	4.94	21.2
3	Craig Kimbrel C. Kimbrel	16.66	62.2	2012	ATL	63	3	1	.750	0	1.01	116	0	0	42	27	7	7	3	14	2	56	0	231	5	2.01	8.29	0.43	50.2	6.1	0.65	.250	1.01	26	0.77	20	3.88	21.6
4	Craig Kimbrel C. Kimbrel	16.43	69.0	2017	BOS	67	5	0	1.000	0	1.43	126	0	0	35	33	11	11	6	14	4	51	0	254	5	1.83	9.00	0.78	49.6	5.5	0.68	.260	1.43	31	1.41	32	4.30	22.7
5	Josh Hader J. Hader	16.41	75.2	2019	MIL	61	3	5	.375	0	2.62	138	0	0	37	41	22	24	15	20	4	46	2	289	0	2.38	6.90	1.78	47.8	6.9	0.81	.232	2.85	59	3.08	70	4.88	11.3
6	Aroldis Chapman A. Chapman	16.31	51.1	2018	NYY	55	3	0	1.000	0	2.45	93	0	0	32	24	14	15	2	30	5	43	0	212	9	5.26	3.10	0.35	43.9	14.2	1.05	.268	2.63	58	2.08	48	4.21	12.5
7	Félix Bautista F. Bautista	16.23	61.0	2023	BAL	56	8	2	.800	0	1.48	110	0	0	33	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	35	1.87	45	4.43	16
8	Kenley Jansen K. Jansen	16.10	53.2	2011	LAD	51	2	1	.667	0	2.85	96	0	0	5	30	17	17	3	26	2	13	0	218	0	4.36	3.69	0.50	44.0	11.9	1.04	.297	2.85	79	1.73	46	5.03	12.5
9	Carlos Marmol C. Marmol	15.99	77.2	2010	CHC	77	2	3	.400	0	2.55	138	0	0	38	40	22	23	1	52	8	70	4	332	2	6.03	2.65	0.12	41.6	15.7	1.18	.293	2.67	62	2.00	50	4.64	17.4
10	Aroldis Chapman A. Chapman	15.89	58.1	2023	TOT	61	6	4	.600	0	3.09	103	0	0	6	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	71	2.51	58	5.71	11.7
11	Aroldis Chapman A. Chapman	15.83	63.2	2013	CIN	68	4	5	.444	0	2.54	112	0	0	38	37	18	18	7	29	3	55	0	258	6	4.10	3.86	0.99	43.4	11.2	1.04	.280	2.54	67	2.46	64	5.23	9.8
12	Josh Hader J. Hader	15.82	81.1	2018	MIL	55	6	1	.857	0	2.43	143	0	0	12	36	22	23	9	30	1	14	0	306	0	3.32	4.77	1.00	46.7	9.8	0.81	.220	2.55	60	2.22	55	3.98	16.6
13	Aroldis Chapman A. Chapman	15.74	66.1	2015	CIN	65	4	4	.500	0	1.63	116	0	0	33	43	12	13	3	33	5	54	1	278	7	4.48	3.52	0.41	41.7	11.9	1.15	.331	1.76	41	1.94	49	5.83	14.6
14	Josh Hader J. Hader	15.65	58.2	2021	MIL	60	4	2	.667	0	1.23	102	0	0	34	25	8	8	3	24	2	42	0	224	3	3.68	4.25	0.46	45.5	10.7	0.84	.237	1.23	29	1.68	39	3.84	16.8
15	Dellin Betances D. Betances	15.53	73.0	2016	NYY	73	3	6	.333	0	3.08	126	0	0	12	54	25	31	5	28	1	20	0	299	6	3.45	4.50	0.62	42.1	9.4	1.12	.353	3.82	73	1.77	40	6.66	20.7
16	Dellin Betances D. Betances	15.52	66.2	2018	NYY	66	4	6	.400	0	2.70	115	0	0	4	44	20	22	7	26	5	15	2	272	4	3.51	4.42	0.95	42.3	9.6	1.05	.311	2.97	63	2.47	57	5.94	13.5
17	Aroldis Chapman A. Chapman	15.50	56.1	2021	NYY	61	6	4	.600	0	3.36	97	0	0	30	36	21	23	9	38	3	46	2	243	7	6.07	2.55	1.44	39.9	15.6	1.31	.281	3.67	79	3.98	92	5.75	2.1
18	Matt Barnes M. Barnes	15.39	64.1	2019	BOS	70	5	4	.556	0	3.78	110	0	0	4	51	27	29	8	38	2	14	2	285	13	5.32	2.89	1.12	38.6	13.3	1.38	.339	4.06	78	3.26	72	7.13	9
19	Edwin Díaz E. Díaz	15.36	58.0	2019	NYM	66	2	7	.222	0	5.59	99	0	0	26	58	36	36	15	22	4	48	3	254	3	3.41	4.50	2.33	39.0	8.7	1.38	.377	5.59	137	4.50	105	9.00	-1.4
20	Edwin Díaz E. Díaz	15.33	51.2	2016	SEA	49	0	4	.000	0	2.79	88	0	0	18	45	16	16	5	15	3	23	2	217	6	2.61	5.87	0.87	40.6	6.9	1.16	.377	2.79	70	2.04	49	7.84	12.4
21	Aroldis Chapman A. Chapman	15.32	71.2	2012	CIN	68	5	5	.500	0	1.51	122	0	0	38	35	12	13	4	23	4	52	0	276	4	2.89	5.30	0.50	44.2	8.3	0.81	.252	1.63	38	1.54	38	4.40	19.3
22	Nick Anderson N. Anderson	15.23	65.0	2019	TOT	68	5	4	.556	0	3.32	110	0	0	1	52	24	24	8	18	2	3	4	264	4	2.49	6.11	1.11	41.7	6.8	1.08	.349	3.32	79	2.34	54	7.20	14.7
23	Edwin Díaz E. Díaz	15.22	73.1	2018	SEA	73	0	4	.000	0	1.96	124	0	0	57	41	16	17	5	17	6	65	0	280	3	2.09	7.29	0.61	44.3	6.1	0.79	.281	2.09	48	1.60	38	5.03	21.3
24	Craig Kimbrel C. Kimbrel	15.08	59.2	2021	TOT	63	4	5	.444	0	2.26	100	0	0	24	31	15	19	6	23	3	43	1	235	8	3.47	4.35	0.91	42.6	9.8	0.91	.243	2.87	52	2.42	55	4.68	12.6
25	Dellin Betances D. Betances	15.08	59.2	2017	NYY	66	3	6	.333	0	2.87	100	0	0	10	29	19	20	3	44	11	21	0	261	5	6.64	2.27	0.45	38.3	16.9	1.22	.252	3.02	66	3.22	72	4.37	8.2